The Transient High Energy Sky and Early Universe Surveyor ... · Keywords Gamma-ray: bursts Cosmology: observations, dark ages, re-ionization, first stars 1 Introduction The Transient

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The Transient High Energy Sky and Early Universe Surveyor (THESEUS)

Amati, L.; O'Brien, P.; Goetz, D.; Bozzo, E.; Tenzer, C.; Frontera, F.; Ghirlanda, G.; Labanti, C.; Osborne,J. P.; Stratta, G.Total number of authors:212

Published in:Space Science Reviews

Publication date:2020

Document VersionEarly version, also known as pre-print

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Citation (APA):Amati, L., O'Brien, P., Goetz, D., Bozzo, E., Tenzer, C., Frontera, F., Ghirlanda, G., Labanti, C., Osborne, J. P.,Stratta, G., Tanvir, N., Willingale, R., Attina, P., Campana, R., Castro-Tirado, A. J., Contini, C., Fuschino, F.,Gomboc, A., Hudec, R., ... Zicha, J. (2020). The Transient High Energy Sky and Early Universe Surveyor(THESEUS). Manuscript submitted for publication.

my journal manuscript No.(will be inserted by the editor)

The Transient High Energy Sky and Early Universe Surveyor(THESEUS)

L. Amati · P. O’Brien · D. Gotz · E. Bozzo · C. Tenzer · F. Frontera ·G. Ghirlanda · C. Labanti · J. P. Osborne · G. Stratta · N. Tanvir · R. WillingaleP. Attina · R. Campana · A.J. Castro-Tirado · C. Contini · F. Fuschino ·A. Gomboc · R. Hudec · P. Orleanski · E. Renotte · T. Rodic · Z. Bagoly ·A. Blain · P. Callanan · S. Covino · A. Ferrara · E. Le Floch · M. Marisaldi ·S. Mereghetti · P. Rosati · A. Vacchi · P. D’Avanzo · P. Giommi · A. Gomboc ·S. Piranomonte · L. Piro · V. Reglero · A. Rossi · A. Santangelo · R. Salvaterra ·G. Tagliaferri · S. Vergani · S. Vinciguerra · M. Briggs · E. Campolongo ·R. Ciolfi · V. Connaughton · B. Cordier · B. Morelli · M. Orlandini · C. Adami ·A. Argan · J.-L. Atteia · N. Auricchio · L. Balazs · G. Baldazzi · S. Basa ·R. Basak · P. Bellutti · M. G. Bernardini · G. Bertuccio · J. Braga · M. Branchesi ·S. Brandt · E. Brocato · C. Budtz-Jorgensen · A. Bulgarelli · L. Burderi · J. Camp ·S. Capozziello · J. Caruana · P. Casella · B. Cenko · P. Chardonnet · B. Ciardi ·S. Colafrancesco · M. G. Dainotti · V. D’Elia · D. De Martino · M. De Pasquale ·E. Del Monte · M. Della Valle · A. Drago · Y. Evangelista · M. Feroci · F. Finelli ·M. Fiorini · J. Fynbo · A. Gal-Yam · B. Gendre · G. Ghisellini · A. Grado ·C. Guidorzi · M. Hafizi · L. Hanlon · J. Hjorth · L. Izzo · L. Kiss · P. Kumar ·I. Kuvvetli · M. Lavagna · T. Li · F. Longo · M. Lyutikov · U. Maio · E. Maiorano ·P. Malcovati · D. Malesani · R. Margutti · A. Martin-Carrillo · N. Masetti ·S. McBreen · R. Mignani · G. Morgante · C. Mundell · H. U. Nargaard-Nielsen ·L. Nicastro · E. Palazzi · S. Paltani · F. Panessa · G. Pareschi · A. Pe’er ·A. V. Penacchioni · E. Pian · E. Piedipalumbo · T. Piran · G. Rauw · M. Razzano ·A. Read · L. Rezzolla · P. Romano · R. Ruffini · S. Savaglio · V. Sguera ·P. Schady · W. Skidmore · L. Song · E. Stanway · R. Starling · M. Topinka ·E. Troja · M. van Putten · E. Vanzella · S. Vercellone · C. Wilson-Hodge ·D. Yonetoku · G. Zampa · N. Zampa · B. Zhang · B. B. Zhang · S. Zhang ·S.-N. Zhang · A. Antonelli · F. Bianco · S. Boci · M. Boer · M. T. Botticella ·O. Boulade · C. Butler · S. Campana · F. Capitanio · A. Celotti · Y. Chen ·M. Colpi · A. Comastri · J.-G. Cuby · M. Dadina · A. De Luca · Y.-W. Dong ·S. Ettori · P. Gandhi · E. Geza · J. Greiner · S. Guiriec · J. Harms · M. Hernanz ·A. Hornstrup · I. Hutchinson · G. Israel · P. Jonker · Y. Kaneko · N. Kawai ·K. Wiersema · S. Korpela · V. Lebrun · F. Lu · A. MacFadyen · G. Malaguti ·L. Maraschi · A. Melandri · M. Modjaz · D. Morris · N. Omodei · A. Paizis ·P. Pata · V. Petrosian · A. Rachevski · J. Rhoads · F. Ryde · L. Sabau-Graziati ·N. Shigehiro · M. Sims · J. Soomin · D. Szecsi · Y. Urata · M. Uslenghi ·L. Valenziano · G. Vianello · S. Vojtech · D. Watson · J. Zicha

Received: date / Accepted: date

L. AmatiINAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna, ItalyE-mail: [email protected]

P. O’BrienDepartment of Physics and Astronomy, University of Leicester,Leicester LE1 7RH, UK

D. GotzIRFU/Departement d’Astrophysique, CEA, Universite Paris-Saclay,F-91191, Gif-sur-Yvette, France

E. BozzoDepartment of Astronomy, University of Geneva, ch. d’Ecogia 16,CH-1290 Versoix, Switzerland

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Abstract THESEUS is a space mission concept aimed atexploiting Gamma-Ray Bursts for investigating the early Uni-verse and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals willbe achieved through a unique combination of instrumentsallowing GRBs and X-ray transients detection over a broadFOV (more than 1sr) with 0.5-1 arcmin localization, an en-ergy band extending from several MeVs down to 0.3 keVand high sensitivity to transient sources in the soft X-ray do-main, as well as on-board prompt (few minutes) follow-upwith a 0.7 m class IR telescope with both imaging and spec-troscopic capabilities. THESEUS will be perfectly suited foraddressing the main open issues in cosmology such as, e.g.,star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift ∼10, signa-tures of Pop III stars, sources and physics of re-ionization,and the faint end of the galaxy luminosity function. In addi-tion, it will provide unprecedented capability to monitor theX-ray variable sky, thus detecting, localizing, and identify-ing the electromagnetic counterparts to sources of gravita-tional radiation, which may be routinely detected in the late’20s / early ’30s by next generation facilities like aLIGO/

aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUSwill also provide powerful synergies with the next gener-ation of multi-wavelength observatories (e.g., LSST, ELT,SKA, CTA, ATHENA).

Keywords Gamma-ray: bursts · Cosmology: observations,dark ages, re-ionization, first stars

1 Introduction

The Transient High Energy Sky and Early Universe Sur-veyor (THESEUS) is a space mission concept developed bya large international collaboration in response to the callsfor M-class missions by the European Space Agency (ESA).THESEUS is designed to vastly increase the discovery spaceof high energy transient phenomena over the entirety of cos-mic history (Fig. 1 and Fig. 2). Its driving science goalsaim at finding answers to multiple fundamental questions ofmodern cosmology and astrophysics, exploiting the missionunique capability to: a) explore the physical conditions ofthe Early Universe (the cosmic dawn and re-ionization era)by unveiling the Gamma-Ray Burst (GRB) population in thefirst billion years; b) perform an unprecedented deep mon-itoring of the soft X-ray transient Universe, thus providing

C. TenzerInstitut fur Astronomie und Astrophysik, Abteilung Hochenergieastro-physik, Kepler Center for Astro and Particle Physics, Eberhard KarlsUniversitat, Sand 1, D 72076 Tubingen, Germany

Full author list and affiliations are provided at the end of thepaper.

Fig. 1 Gamma-Ray Bursts in the cosmological context and the roleof THESEUS (adapted from a picture by the NASA/WMAP ScienceTeam).

a fundamental synergy with the next-generation of gravita-tional wave and neutrino detectors (multi-messenger astro-physics), as well as the large electromagnetic (EM) facilitiesof the next decade.

The most critical THESEUS targets, i.e. GRBs, are uniqueand powerful tools for cosmology, especially because of theirhuge luminosities, mostly emitted in X- and gamma-rays,their redshift (z) distribution (extending at least to z∼10),and their association with the explosive death of massivestars. In particular, GRBs represent a unique tool to studythe early Universe up to the re-ionization era. To date, thereis no consensus on the dominant sources of re-ionization,and GRB progenitors and their hosts are very good repre-sentatives of the massive stars and star-forming galaxies thatmay have been responsible for this cosmic phase change.A statistical sample of high-z GRBs (between 30 and 80 atz>6, see Fig. 2) can provide fundamental information suchas: measuring independently the cosmic star formation rate,even beyond the limits of current and future galaxy surveys,the number density and properties of low-mass galaxies, theneutral hydrogen fraction, and the escape fraction of UVphotons from high-z galaxies. Even JWST and ELTs sur-veys, in the 2020s, will not be able to probe the faint end ofthe galaxy luminosity function (LF) at high redshifts (z>6-8). The first generation of metal-free stars (the so-called PopIII stars) and the second generation of massive, metal-poorstars (the so-called Pop II stars) can result in powerful GRBs(see, e.g., Meszaros and Rees 2010) that thus offer a directroute to identify such elusive objects (even JWST will not beable to detect them directly) and study the galaxies in which

THESEUS 3

Fig. 2 The yearly cumulative distribution of GRBs with redshift determination as a function of the redshift for Swift and THESEUS (see detailsin Sect. 2.1). We note that these predictions are conservative in so far as they reproduce the current GRB rate as a function of redshift. However,with our sensitivity, we can detect a GRB of Eiso ∼ 1053 erg (corresponding to the median of the GRB radiated energy distribution) up to z=12.Indeed, our poor knowledge of the GRB rate-SFR connection does not preclude the existence of a sizable number of GRBs at such high redshifts,in keeping with recent models of Pop III stars.

they are hosted. Even indirectly, the role of Pop III stars inenriching the first galaxies with metals can be studied bylooking to the absorption features of Pop II GRBs blowingout in a medium enriched by the first Pop III supernovae (Maet al. 2017). Moreover, both Pop III and massive Pop II starsmay have contributed to the reionization of the Universe dueto their intensive ionizing radiation (Yoon et al. 2012; Szecsiet al. 2015), thus detecting their final explosion will help usbetter understand our cosmic history.

Besides high-redshift GRBs, THESEUS will serendipi-tously detect and localize during regular observations a largenumber of X-ray transients and variable sources (Fig. 3 andFig. 4), collecting also prompt follow-up data in the IR (seeTables 1 and 2). These observations will provide a wealth ofunique science opportunities, by revealing the violent Uni-verse as it occurs in real-time, exploiting an all-sky X-raymonitoring of extraordinary grasp and sensitivity carried outat high cadence. THESEUS will be able to locate and iden-tify the electromagnetic counterparts to sources of gravita-tional radiation and neutrinos, which will be routinely de-tected in the late ’20s / early ’30s by next generation fa-cilities like aLIGO/ aVirgo, eLISA, ET, and Km3NET. Inaddition, the provision of a high cadence soft X-ray moni-toring capability in the 2020s together with a 0.7 m IRT inorbit will enable a strong synergy with transient phenomenaobserved with the large facilities that will be operating inthe EM domain (e.g., ELT, SKA, CTA, JWST, ATHENA).The large number of GRBs found in the survey will per-mit unprecedented insights in the physics and progenitors ofthese events and their connection with peculiar core-collapseSNe. THESEUS will also substantially increase the detec-

tion rate and characterization of sub-energetic GRBs andX-Ray Flashes. Monitoring observations of bright and faintX-ray objects will be routinely carried out, and a dramaticincrease in the rate of discovery of high-energy transientsources over the whole sky is certainly expected, includ-ing supernova shock break-outs, black hole tidal disruptionevents, and magnetar flares.

The primary scientific goals of the mission thus addressthe Early Universe ESA Cosmic Vision theme “How did theUniverse originate and what is it made of?” and, specifically,the sub-themes: 4.1) Early Universe, 4.2) The Universe tak-ing shape, and 4.3) The evolving violent Universe. Theyalso have a relevant impact on the 3.2 themes: “The Grav-itational Wave Universe” and “The Hot and Energetic Uni-verse”. In addition, THESEUS will also automatically en-able excellent observatory science opportunities, thus allow-ing a strong community involvement. It is worth remarkingthat THESEUS will have survey capabilities for high-energytransient phenomena complementary to the Large SynopticSurvey Telescope (LSST) in the optical. Their joint avail-ability at the end of the next decade would enable a remark-able scientific synergy between them.

The THESEUS scientific goals related to the full explo-ration of the early Universe requires requires the detectionof many tens of GRBs from the first billion years (about30-80), around ten times the number currently known at aredshift z>6 (Fig. 2). This is well beyond the capabilities ofcurrent and near future GRB detectors (Swift/BAT, the mostsensitive one, has detected only very few GRBs above z=6 in10 yrs). As supported by intensive simulations performed byus and other works in the literature (see, e.g., Ghirlanda et al.

4 Amati et al.

Fig. 3 By the end of the ’20s, if Einstein Telescope will be a single detector, almost no directional information will be available for GW sources,and a GRB-localising satellite will be essential to discover EM counterparts. If multiple third-generation detectors will be available, the typicallocalization uncertainties will be comparable to the aLIGO ones. The plot shows the SXI field of view (∼110×30 deg2) superimposed on theprobability skymap of GW151226 observed by aLIGO.

Fig. 4 Sensitivity of the SXI (black curves) and XGIS (red) vs. integration time (only for the instrument central spot). The solid curves assume asource column density of 5×1020 cm−2 (i.e., well out of the Galactic plane and very little intrinsic absorption). The dotted curves assume a sourcecolumn density of 1022 cm−2 (significant intrinsic absorption). The black dots are the peak fluxes for Swift BAT GRBs plotted against T90/2. Theflux in the soft band 0.3-10 keV was estimated using the T90 BAT spectral fit including the absorption from the XRT spectral fit. The red dotsare those GRBs for which T90/2 is less than 1 s. The green dots are the initial fluxes and times since trigger at the start of the Swift XRT GRBlight-curves. The horizontal lines indicate the duration of the first time bin in the XRT light-curve. The various shaded regions illustrate variabilityand flux regions for different types of transients and variable sources.

2015), the required substantial increase of high-z GRBs im-plies both an increase of ∼1 order of magnitude in sensitivityand an extension of the detector passband down to the softerX-rays (0.5-1 keV). Such capabilities must be provided overa broad field of view (∼1 sr) with a source location accuracy<2 arcmin, in order to allow efficient counterpart detection,

on-board spectroscopy and redshift measurement, as well asoptical and IR follow-up observations.

The required THESEUS performances can be best ob-tained by including in the payload a monitor based on thelobster-eye telescope technology, capable of focusing softX-rays in the 0.3-6 keV energy band over a large FOV. Suchinstrumentation has been under development for several years

THESEUS 5

Fig. 5 THESEUS Satellite Baseline Configuration and Instrument suite accommodation.

at the University of Leicester, has a high TRL level (e.g.,BepiColombo, SVOM) and can perform all-sky monitor-ing in the soft X-rays with an unprecedented combinationof FOV, source location accuracy (<1-2 arcmin) and sensi-tivity. An on-board infrared telescope of the 0.5-1 m classis also needed, together with the fast slewing capability ofthe spacecraft (e.g., several degrees per minute), in order toprovide prompt identification of the GRB optical/IR coun-terpart, refinement of the position down to ∼arcsec preci-sion (thus enabling follow-up with the largest ground andspace observatories), on-board redshift determination andspectroscopy of the counterpart and of the host galaxy. Thiscapability will add considerably to the science value of ATHENAby providing a sample of known high-redshift bursts withwhich it will be able to sample the intervening WHIM.

The telescope may also be used for multiple observatoryand survey science goals. Finally, the inclusion in the pay-load of a broad field of view hard X-ray detection systemcovering the same monitoring FOV as the lobster-eye tele-scopes and extending the energy band from few keV up toseveral MeV will increase significantly the capabilities ofthe mission. As the lobster-eye telescopes can be triggeredby many classes of transient phenomena (e.g., flare stars, X-ray bursts, etc), the hard X-ray detection system provides anefficient means to identify real GRBs and detect other tran-sient sources (e.g., soft Gamma-repeaters). The joint datafrom the three instruments will characterize transients interms of luminosity, spectra and timing properties over a

broad energy band, thus getting fundamental insights intotheir physics. In summary, the foreseen payload of THE-SEUS includes the following instrumentation:

– Soft X-ray Imager (SXI, 0.3-6 keV): a set of 4 lobster-eye telescopes units, covering a total FOV of ∼1 sr withsource location accuracy <1-2 arcmin;

– InfraRed Telescope (IRT, 0.7-1.8 µm): a 0.7 m class IRtelescope with 10×10 arcmin FOV, for fast response,with both imaging and spectroscopy capabilities;

– X-Gamma ray Imaging Spectrometer (XGIS, 2 keV-20 MeV):a set of coded-mask cameras using monolithic X-gammaray detectors based on bars of Silicon diodes coupledwith CsI crystal scintillator, granting a ∼1.5 sr FOV, asource location accuracy of ∼5 arcmin in 2-30 keV andan unprecedently broad energy band.

In Fig. 5 we show a sketch of the THESEUS baselineconfiguration and instrument suite accommodation as pro-posed for ESA/M5. The mission profile also includes: an on-board data handling units (DHUs) system capable of detect-ing, identifying and localizing likely transients in the SXIand XGIS FOV; the capability of promptly (within a fewtens of seconds at most) transmitting to ground the triggertime and position of GRBs (and other transients of interest);a spacecraft slewing capability of ∼5-10 deg/min. The base-line launcher/orbit configuration is a launch with Vega-C toa low inclination low Earth orbit (LEO, ∼600 km, <5 deg),which has the unique advantages of granting a low and sta-ble background level in the high-energy instruments, allow-

6 Amati et al.

ing the exploitation of the Earth’s magnetic field for space-craft fast slewing and facilitating the prompt transmission oftransient triggers and positions to the ground.

This article provides an extensive review of the THE-SEUS science case, scientific requirements, proposed instru-ments, mission profile, expected performances, as proposedin response to the ESA Call for M5 mission within the Cos-mic Vision Programme. The current THESEUS collabora-tion involves more than 100 scientists from all over the world.The payload consortium is composed by several Europeancountries, with a main role of Italy, UK, France, Germany,and Switzerland. Significant contributions are planned bySpain, Belgium, Czech Republic, and Poland. Additionalcontributions have been proposed by Ireland, Hungary, Slove-nia, and Albania. International contributions are being dis-cussed in several extra-European countries, including USA,Brazil, and China (already involved in the mission study).OHB Italia and GP Advanced Projects (Italy) kindly con-tributed to the study and definition of the technical aspectsof the THESEUS mission concept.

2 Science case

The THESEUS specific science objectives can be summa-rized as follows:

A. Explore the physical conditions of the Early Universe(cosmic dawn and reionization era) by unveiling a com-plete census of the GRB population in the first billionyears. This will be achieved by:– Performing studies of the global star formation his-

tory of the Universe up to z∼10 and possibly beyondwith an unprecedented sensitivity for the detectionof GRBs at high redshifts;

– Detecting and studying the primordial (Pop III) andsubsequent (Pop II) star populations: when did theearliest stars form and how did they influence theirenvironments?

– Investigating the re-ionization epoch by determiningthe physical properties of the the interstellar medium(ISM) and the intergalactic medium (IGM) up to z ∼8-10: how did re-ionization proceed as a function ofenvironment, and was radiation from massive starsits primary driver? How did cosmic chemical evolu-tion proceed as a function of time and environment?

– Investigating the properties of the early galaxies anddetermining their star formation properties in the re-ionization era.

B. Perform an unprecedented deep monitoring of the X-raytransient Universe in order to:– Locate and identify the electromagnetic counterparts

to sources of gravitational radiation and neutrinos,which may be routinely detected in the late ’20s /

early ’30s by next generation facilities like aLIGO/

aVirgo, eLISA, ET, or Km3NET;– Provide real-time triggers and accurate (∼1 arcmin

within a few seconds; ∼1 arcsec within a few min-utes) locations of (long/short) GRBs and high-energytransients for follow-up with next-generation optical-NIR (ELT, JWST if still operating), radio (SKA), X-rays (ATHENA), and TeV (CTA) telescopes;

– Provide a fundamental step forward in the compre-hension of the physics of various classes of Galac-tic and extra-Galactic transients, e.g.: tidal disruptionevents (TDE), magnetars/SGRs, SN shock break-outs,Soft X-ray Transients, thermonuclear bursts from ac-creting neutron stars, Novae, dwarf novae, stellar flares,AGNs and Blazars;

– Fill the present gap in the discovery space of newclasses of high-energy transient events, thus provid-ing unexpected phenomena and discoveries.

Below, we detail the THESEUS science case and de-scribe the rich observatory science that will come out as a re-sult of meeting the primary mission requirements. We showthat the unique capabilities of this mission will enlarge sev-eral research fields, and will be beneficial for multiple sci-entific communities.

2.1 Exploring the Early Universe with Gamma-Ray Bursts

A major goal of contemporary astrophysics and cosmologyis to achieve a broad understanding of the formation of thefirst collapsed structures (Pop III and early Pop II stars, blackholes and galaxies) during the first billion years in the life ofthe universe. This is intimately connected to the reioniza-tion of the IGM and build-up of global metallicity. The lat-ter is very poorly constrained, and even in the JWST era willrely on crude emission line diagnostics for only the bright-est galaxies. Regarding reionization, measurements of theThomson scattering optical depth to the microwave back-ground by the Planck satellite now suggest it substantiallyoccurred in the redshift range z∼7.8-8.8 (see, e.g., PlanckCollaboration et al. 2016), whereas the observations of theGunn-Peterson trough in the spectra of distant quasars andgalaxies indicate it was largely finished by z∼6.5 (see, e.g.,Caruana et al. 2012, 2014; Schenker et al. 2014). Statisti-cal measurements of the fluctuations in the redshifted 21 cmline of neutral hydrogen by experiments such as LOFAR andSKA are expected to soon provide further constraints on thetime history (see, e.g., Patil et al. 2014). The central ques-tion, however, remains whether it was predominantly radi-ation from massive stars that both brought about and sus-tained this phase change, or whether more exotic mecha-nisms must be sought.

THESEUS 7

Fig. 6 Redshift distribution of GRBs (credits Berger, Harvard/CfA).

Solving this problem largely splits into two subsidiaryissues: how much massive star formation was occurring asa function of redshift? And, on average, what proportion ofthe ionizing radiation produced by these massive stars man-ages to escape from the immediate environs of their hostgalaxies? An answer to the former question can be extrap-olated based on observed candidate z >7 galaxies found inHubble Space Telescope (HST) deep fields, but there are twovery significant uncertainties. The first is the completenessand cleanliness of the photometric redshift samples at z >7,while the second is the poorly constrained form and faint-end behaviour of the galaxy LF (at stellar masses .108 M),especially since galaxies below the HST detection limit verylikely dominate the star formation budget. Even though someconstraints on fainter galaxies can be obtained through ob-servations of lensing clusters (see, e.g., Atek et al. 2015;Vanzella et al. 2017a,b), which will be improved further byJWST, simulations suggest star formation was still likelyoccurring in considerably fainter systems (Liu et al. 2016).The second question concerns the Lyman continuum escapefraction and it is even more difficult to be answered, sincethis parameter cannot be determined directly at high red-shifts and lower redshift studies have generally found ratherlow values of the escape fraction fesc of only a few per-cent (Grazian et al. 2017). This is likely insufficient to drivereionization unless fesc increases significantly at early timesand/or for smaller galaxies (see, e.g., Robertson et al. 2013).Recently, an analysis of a sample of GRB afterglow spec-tra in the range 1.6 < z < 6.7 found a 98% upper limit tothe average ionizing escape fraction of 1.5% (Tanvir et al.submitted).

These questions may be answered by the help of GRBsand their host galaxies. With the help of THESEUS, we cancombine information on the cold/warm ISM gas of faint starforming galaxies (through the afterglow spectroscopy) withthat on the emission properties of the continuum and ionised

gas of the galaxy (through follow-up observations). There-fore, we will be able to probe star formation efficiency, es-cape fraction, metal enrichment and galaxy evolution dur-ing, and even preceding, the epoch of reionization. This uniquecontribution will be one of the most important that THE-SEUS will provide.

Global star formation from GRB rate as a function of red-shiftLong-duration GRBs are produced by massive stars, and sotrack star formation, and in particular the populations ofUV-bright stars responsible for the bulk of ionizing radia-tion production. Although there is evidence at low redshiftthat GRBs are disfavoured in high metallicity environments,since high redshift star formation is predominantly at lowmetallicity (see, e.g., Salvaterra et al. 2013; Perley et al.2016; Vergani et al. 2017) it is likely that the GRB rate tomassive star formation rate ratio is approximately constantbeyond z '3. Thus simply establishing the GRB N(z), andaccounting for the instrumental selection function, providesa direct tracer of the global star formation rate density as afunction of cosmic time (Fig. 6). How selection bias is takeninto account is relevant for a correct evaluation of the starformation rate (Petrosian et al. 2015; Dainotti et al. 2015).Analyses of this sort have consistently pointed to a higherSFR density at redshifts z > 6 than traditionally inferredfrom UV rest-frame galaxy studies (see Fig. 7 left), whichrely on counting star-forming galaxies and attempting to ac-count for galaxies below the detection threshold. Althoughthis discrepancy has been alleviated by the growing realisa-tion of the extremely steep faint-end slope of the galaxy LFat z > 6, it still appears that this steep slope must continue tovery faint magnitudes, MAB ' −11, in order to provide con-sistency with GRB counts and indeed to achieve reionization(something that can only be quantified via a full census ofthe GRB population).

The high-z galaxy luminosity functionThe shape and normalisation of the high-redshift galaxy lu-minosity function is a key issue for understanding reioniza-tion since, to the depth achieved in the Hubble Ultra-deepField (HUDF), it appears that the faint-end of the LF atz > 6 approaches a power-law of slope α = 2. Thus thevalue of the total luminosity integral depends sensitively onthe choice of the low-luminosity cut-off (and indeed the as-sumption of continued power-law form for the LF). By con-ducting deep searches for the hosts of GRBs at high-z wecan directly estimate the ratio of star formation occurring indetectable and undetectable galaxies, with the sole assump-tion that the GRB-rate is proportional to the star formationrate (see Fig. 7 left). Although currently limited by small-number statistics, the early application of this technique has

8 Amati et al.

confirmed that the majority of the star formation at z & 6 oc-curred in galaxies below the effective detection limit of HST(Tanvir et al. 2012; McGuire et al. 2016). Since the exact po-sition and redshift of the galaxy is known via the GRB after-glow, follow-up observations are more efficient than equiv-alent deep field searches for Lyman-break galaxies.

The build-up of metals, molecules and dustBright GRB afterglows with their intrinsic power-law spec-tra provide ideal backlights for measuring not only the hy-drogen column, but also obtaining exquisite abundances andgas kinematics probing to the hearts of their host galaxies(see Fig. 7 right and, e.g., Hartoog et al. 2015). Further, theimprint of the local dust law, and in some cases observa-tion of H2 molecular absorption, provides further detailedevidence of the state of the host ISM (see, e.g., Friis et al.2015). They can thus be used to monitor cosmic metal en-richment and chemical evolution to early times, and searchfor evidence of the nucleosynthetic products of even earliergenerations of stars. In the late 2020s, taking advantage ofthe availability of 30 m class ground-based telescopes (anddefinitely ATHENA), superb abundance determinations willbe possible through simultaneous measurement of metal ab-sorption lines and modelling the red-wing of Ly-alpha todetermine host HI column density, potentially even manydays after the burst (see Fig. 9). We emphasize that usingthe THESEUS on-board NIR spectroscopy capabilities (seeSect. 4 and Fig. 37) will provide the redshifts and luminos-ity measurements that are essential to optimise the time-critical follow-up observations using the highly expensivenext-generation facilities, allowing us to select the highestpriority targets and use the most appropriate telescope andinstrument.

Topology of reionizationIt is expected that reionization should proceed in a patchyway, with ionized bubbles being created first around the high-est density peaks where the first galaxies occur, expandingand ultimately filling the whole IGM. The topology of thegrowing network of ionized regions reflects the characterof the early structure formation and the ionizing radiationfield. With high-S/N afterglow spectroscopy, the Ly-alphared damping wing can be decomposed into contributionsdue to the host galaxy and the IGM. The latter provides thehydrogen neutral fraction and so measures the progress ofreionization local to the burst. With samples of several tensof GRBs at z & 7 − 8, we can begin to statistically investi-gate the average and variance of the reionization process asa function of redshift (see, e.g., McQuinn et al. 2008).

Population III starsThe first stars in the Universe are supposed to be very mas-sive and completely metal-free. However, no direct detec-tion of such objects, called Pop III stars, has been made so

far. Thus, our theoretical understanding of the structure andevolution, as well as the nature of the final explosion, of thefirst stars still relies on many assumptions. Stellar evolutionpredicts that long GRBs may be expected from fast rotating,single Pop III stars with an initial mass of 10-80 M (Yoonet al. 2012). Above this mass limit, a Pop III star may ex-plode due to pair-instability as a Superluminous Supernova,or not explode at all, falling back directly to a black hole(Heger et al. 2003). When a Pop III star explodes it changesthe chemical composition of its environment, from whichthe second stars are formed. These ideas must be examined.Also, having a full census of GRBs with Pop III progenitorshas the potential to provide an unprecedented enhancementof our knowledge of stellar evolution.

The same is true for the subsequent generations of starsin the early Universe: low-metallicity massive stars, or PopII stars, have been simulated using our best theoretical ap-proach, but observational evidences are needed to confirmthe theory (Szecsi et al. 2015). In particular, one of the mostpromising evolutionary channels leading to fast rotating he-lium stars (the so-called “chemically homogeneous evolu-tion”) still needs to be tested observationally. These fast ro-tating helium stars at low metallicity (or TWUIN stars; Szecsiet al. 2015) are predicted to be progenitors of long GRBs ifsingle (Yoon et al. 2006) or progenitors of short GRBs if ina close binary system (Marchant et al. 2017; Szecsi 2017).The detection of GRBs with Pop III or Pop II progenitorsprovides important insights on stellar astrophysics.

Both Pop III and massive Pop II stars are expected toemit a large number of ionizing photons and thus to con-tribute to the ionization of their surroundings. Despite beingpotentially crucial in understanding cosmic reionization, thepredictions of the models are still weighted with uncertain-ties due to scarce observational constraints on metal-free andmetal-poor massive stars. We also expect that interpretingthe data output of THESEUS will bring together scientistsworking on the so-far mostly unrelated fields of reionizationhistory and stellar evolution.

In addition to possible evidence for population III chem-ical enrichment, it has been argued that Pop III stars mayalso produce collapsar-like jetted explosions, which are likelyto be of longer duration than typical long-GRBs, and may bedetected through surveys with longer dwell times (Meszarosand Rees 2010). To date, no direct evidence of this connec-tion has been observationally established. The multiwave-length properties of GRBs with a Pop-III progenitor are onlypredicted on the expected large masses and zero metallicityof these stars. Even the detection of a single GRB from aPop III progenitor would put fundamental constraints on theunknown properties of the first stars.

The role of THESEUSOur detailed simulations indicate that THESEUS will de-

THESEUS 9

Fig. 7 Left: Star Formation Rate density as a function of redshift from Kistler et al. (2013). Low-z data (circles) are from Hopkins and Beacom(2006), while diamonds are from Kistler et al. (2013). The open squares show the result of integrating the Lyman break galaxy UV luminosityfunctions down to the lowest measured magnitude in optical, while the solid squares represent the results obtained by stopping the integration atan optical magnitude of -10. The Salpeter initial mass function (IMF) is assumed in all cases. The plot also show with dotted lines the critical ρ∗

from Madau et al. (1999) for a ratio between the clumping factor and the escape fraction of 40, 30, and 20 (top to bottom). Right: Absorption-linebased metallicities [M/H] as a function of redshift of Damped Lyman-α absorbers, for GRB-DLAs (blue symbols) and emission line metallicitiesfor GRB hosts (red symbols, adapted from Sparre et al. 2014). GRBs are essential to probe the evolution of ISM metallicities in the first billionyears of cosmic history.

tect between 30 and 80 GRBs at z > 6 over a three yearmission, with between 10 and 25 of these at z > 8 (and sev-eral at z > 10). The on-board follow-up capability meansthat redshifts are estimated for the large majority of these,and powerful next generation ground- and space-based tele-scopes available in this era will lead to extremely deep hostsearches and high-S/N afterglow spectroscopy for many ofthem (e.g., using JWST, if still operating, ELT, WFIRST,EUCLID, ATHENA, etc.). To illustrate the potential of sucha sample, we simulate in Fig. 8 the precision in constrainingthe product of the UV luminosity density and average es-cape fraction, ρUV fesc, that would be obtained with 25 GRBsat 7 < z < 9 having high-S/N afterglow spectroscopy and(3 hr) JWST depth host searches (for definiteness the ρUV

axis corresponds to z = 8). This will provide a much cleareranswer to the question of whether stars were the dominantcontributors to reionization. In addition, this sample will al-low us to map abundance patterns across the whole rangeof the star forming galaxies in the early universe, providingmultiple windows on the nature of the first generations ofstars.

The great value of THESEUS to ATHENAAs an example of the great relevance of THESEUS in thecontext of the next generation large facilities (e.g., SKA,CTA, ELT, ATHENA), we highlight here the THESEUS syn-ergy with ATHENA. Two of the primary science goals forATHENA are: (1) Locate the missing baryons in the Uni-verse by probing the Warm Hot Intergalactic Medium (the

WHIM); this requires about 10 bright GRBs per year. (2)Probe the first generation of stars by finding high redshiftGRB; this requires about 5 high-redshift GRBs per year.THESEUS will enable ATHENA to achieve these goals bygreatly increasing the rate of GRBs found per year with goodlocalisations and redshifts. The X-ray band of the THESEUSSXI (see Sect. 4.1) will find a greater proportion of high-redshift GRBs than previous missions. Hence THESEUSwill: (1) localise bright GRBs with sufficient accuracy us-ing the SXI to enable a rapid repointing of the ATHENAX-IFU for X-ray spectroscopy of the WHIM, and (2) findhigh-redshift GRBs using the SXI and XGIS and accuratelylocalising them using the IRT for redshift determination on-board and on the ground to provide reliable high-redshifttargets for ATHENA. Many of the other transients found byTHESEUS, such as tidal disruption events and flaring bina-ries, will also be high-value targets for ATHENA.

2.2 Gravitational wave sources and multi-messengerastrophysics

The launch of THESEUS will coincide with a golden era ofmulti-messenger astronomy. With the first detection of grav-itational waves (GWs) by Advanced detectors (Abbott et al.2016b,a), a new window on the Universe has been opened.By the end of the present decade, the sky will be routinelymonitored by a network of second generation GW detectors,an ensemble of Michelson-type interferometers, composed

10 Amati et al.

Fig. 8 The UV luminosity density from stars at z ' 8 and aver-age escape fraction 〈 fesc〉 are insufficient to sustain reionization un-less the galaxy luminosity function steepens to magnitudes fainter thanMUV = −13 (grey hatched region), and/or 〈 fesc〉 is much higher thanthat typically found at z ' 3 (grey shaded region). Even in the late2020s, 〈 fesc〉 at these redshifts will be largely unconstrained by directobservations. The green contours show the 1 and 2-σ expectations fora sample of 25 GRBs at z ' 7 − 9 for which deep spectroscopy pro-vides the host neutral column and deep imaging constrains the fractionof star formation occurring in hosts below the JWST limit (Robert-son et al. 2013). The input parameters were log10(ρUV) = 26.44 and〈 fesc〉 = 0.23, close to the (red) borderline for maintaining reionizationby stars.

by the two Advanced LIGO (aLIGO) detectors in the USAand by Advanced Virgo (aVirgo) in Italy, plus ILIGO in In-dia and KAGRA in Japan joining later within a few years.By the first years of 2030, more sensitive third generationGW detectors, such as the Einstein Telescope (ET) and Cos-mic Explorer, are planned to be operative and to provide anincrease of roughly one order of magnitude in sensitivity.

Several of the most powerful transient sources of GWspredicted by general relativity, e.g. binary neutron star (NS-NS) or NS-black hole (BH) mergers (with likely detectionrate of ∼50 per year by the LIGO-Virgo network at designsensitivity; Abadie et al. 2010a), are expected to producebright electromagnetic (EM) signals across the entire EMspectrum and in particular in the X-ray and gamma-ray en-ergy bands, as well as neutrinos. GW detectors have rel-atively poor sky localization capabilities mainly based ontriangulation methods. With three detectors ∼80 square de-grees are achieved; this sky localization accuracy may alsoapply for the third-generation GW detectors by 2030+. By2020+ the second-generation network will count three upto five GW detectors and sky localization will improve to∼1-10 sq. degrees or even less (Klimenko et al. 2011). Suchlocalization uncertainties will be completely covered by the

Fig. 9 Simulated ELT spectrum of a GRB at z=8.2 as discoveredby THESEUS. The S/N provides exquisite abundance determinationsfrom metal absorption lines, while fitting the Ly-a damping wing si-multaneously fixes the IGM neutral fraction and the host HI columndensity, as illustrated by the two extreme models: a pure 100% neu-tral IGM (green) and best-fit host absorption with a fully ionized IGM(red).

THESEUS/SXI FoV (see Fig. 10). In parallel to these ad-vancements, IceCube and KM3nNeT and the advent of 10km3 detectors (e.g. IceCube-Gen2, IceCube-Gen2 Collab-oration et al. (2014)) will likely revolutionize neutrino as-trophysics. Neutrino detectors can localize to an accuracyof better than a few sq. degrees but within a smaller vol-ume of the Universe (see, e.g., Santander 2016, and ref-erences therein). In order to maximize the science returnof the multi-messenger investigation it is essential to havean in-orbit trigger and search facility that can either detectan EM signal simultaneous with a GW/neutrino event orrapidly observe with good sensitivity the large error boxesprovided by the GW and neutrino facilities following a trig-ger. These combined requirements are uniquely fulfilled byTHESEUS, which is able to trigger using XGIS or SXI andobserve a very large fraction of the GW/neutrino error boxeswithin an orbit due to the large grasp of the SXI instru-ment (if compared to current generation X-ray facilities suchas Swift, THESEUS/SXI has a grasp of ∼150 times thatof Swift/XRT). For events triggered on-board with XGISor SXI, GW searches can also be carried on the resultantknown sky locations with lower GW detector signal-to-noisethresholds and hence an increased search distance. In 2020ssynergies with the future facilities like JWST, WFIRST, Ein-stein Probe, ATHENA, ELT, TMT, GMT, SKA, CTA, zPTF,and LSST telescopes would leverage significant added value,extending the multi-messenger observations across the wholeEM spectrum. The detection of EM counterparts of GW (orpossibly neutrino) signals will enable a multitude of science

THESEUS 11

Fig. 10 The plot shows the SXI field of view (∼110×30 deg2) super-imposed on a simulated probability skymap of a merger NS-NS systemobserved by aLIGO and advanced Virgo (Singer et al. 2016). The SXIcoverage contains nearly 100% of the total probability.

programmes (see, e.g., Bloom et al. 2009; Phinney 2009)by allowing for parameter constraints that the GW/neutrinoobservations alone cannot fully provide. For example, find-ing a GW/neutrino source EM counterpart in X-rays withTHESEUS/SXI will allow to localize the source with an ac-curacy good enough for optical follow-up and hence to pos-sibly measure its redshift and luminosity. On the other hand,not finding an EM counterpart will constrain merger types(such as BH-BH mergers), magnetic field strengths, and as-trophysical conditions at the time of the merger.

2.2.1 Gravitational wave transient sources

NS-NS /NS-BH mergers: Collimated EM emission from ShortGRBsCompact binary coalescences (CBCs) are among the mostpromising sources of GWs that will be detected in the nextdecade. Indeed, these systems are expected to radiate GWswithin the most sensitive frequency range of ground basedGW detectors (1-2000 Hz), with large GW energy output,of the order of 10−2 Mc2, and gravitational waveforms wellpredicted by General Relativity (see, e.g., Baiotti and Rez-zolla 2017, for a review). The expected rate of NS-NS sys-tems inferred from binary pulsar observations and popula-tion synthesis modeling, is taken to lie between 10 and 1000Gpc−3 yr−1. To date, no NS-BH systems have been observed,but the rate can still be predicted through population synthe-sis, constrained by the observations of NS-NS, to be ∼1-1000 Gpc−3 yr−1 (see, e.g., Abadie et al. 2010a, and refer-ences therein). Mounting indirect evidence associates shortGRB progenitors to CBC systems with at least one neu-tron star (see, e.g., Berger 2014) and provides possible hintsthat the merger of magnetized NS binaries might lead tothe formation of a jet with an opening angle up to of ∼30-40 deg (so far the observed values range from 5-20 deg.;Rezzolla et al. 2011; Troja et al. 2016). GW observationscombined with multi-wavelength follow-up campaigns in

the next years will likely confirm this scenario by simul-taneous detections of CBC-GWs temporally and spatiallyconsistent with short GRBs.

Second-generation GW network can detect NS-NS andNS-BH systems up to 0.4-0.9 Gpc (Abadie et al. 2010a).Since the estimated rate density of detected short GRBs is1-10 Gpc−3 yr−1 (see Wanderman and Piran 2015; Ghirlandaet al. 2016, and references therein) the expected annual rateof on-axis (i.e. face-on) short GRBs with GW counterpartbefore the third generation detectors (see below) is ratherlow, of the order of a few per year or less. However, ina more realistic jet scenario, e.g. a structured jet scenario,prompt GRB photon flux F can vary with observer view-ing angle Θobs. According to Pescalli et al. (2016), F ∼Fcore(Θobs/Θ jet)−s with s > 2, where Fcore is the typical on-axis flux of a short GRB (see Fig. 11). Assuming Fcore ∼10ph cm−2 s−1 (e.g. Narayana Bhat et al. 2016), since shortGRBs have a median redshift of z ∼ 0.5 (Berger 2014), ata distance of ∼ 400 Mpc Fcore would be ∼ 103 ph cm−2

s−1, thus even assuming a steep dependence of F from Θview

(e.g. s = 6) a nearby short GRB can be detected off-axiswith THESEUS/XGIS up to 5Θ jet (we consider the 1 secphoton flux sensitivity of XGIS of 0.2 ph cm−2 s−1 , see Fig.36). Table 1 shows the expected rate of THESESUS/XGISshort GRB detections with a GW counterpart from mergingNS-NS systems (i.e. within the GW detector horizon). Thequoted numbers are obtained by correcting the realistic es-timate of NS-NS merger rate from the jet collimation factorby assuming a jet half-opening angle range of [10-40] deg,and by taking into account the possibility to observe off-axisshort GRBs up to 5Θ jet (see above).

By the time of the launch of THESEUS, gravitational ra-diation from such systems will be likely detectable by third-generation detectors such as the Einstein Telescope (ET) upto redshifts z∼2 or larger (see, e.g., Sathyaprakash et al.2012; Punturo et al. 2010), implying that almost all shortGRBs that THESEUS will detect, i.e., ∼20 short GRBs peryear, will have a detectable GW emission. Indeed, it is likelythat at the typical distances at which ET detects GW events,the only EM counterparts that could feasibly be detected areSGRBs and their afterglows, making the role of THESEUScrucial for multi-messenger astronomy by that time. Almostall short GRBs are accompanied by an X-ray afterglow thatSXI will detect and monitor just after the burst emission.Once localized with SXI, about 40% of detected short GRBsare expected to have a detectable optical/IR counterpart. TheIRT could point at the SXI-localised afterglow within a fewminutes from the trigger. If bright enough, spectroscopicobservations could be performed on-board, thus providingredshift estimates and information on chemical composi-tion of circumburst medium. In addition, precise sky co-ordinates will be disseminated to ground based telescopesto perform spectroscopic observations. Distance measure-

12 Amati et al.

Fig. 11 Top panel: possible geometry of the conical jet, in sphericalcoordinates with the origin at O. The observer is located at (D,Θobs, 0).The jet is moving with Lorentz factor Γ(Θ). Bottom panel: Luminosityfraction (normalized to peak emission) as a function of observer angle,Lobs(Θobs), from Kathirgamaraju et al. (2017). The jet luminosity at∼40 deg is a factor of 300 fainter than that of the jet core. Nevertheless,such a misaligned jet can be detected by a γ-ray instrument if it takesplace within the Advanced LIGO detectability volume.

ments of a large sample of short GRBs combined with theabsolute source luminosity distance provided by the CBC-GW signals can deliver precise measurements of the Hubbleconstant (Schutz 1986), helping to break the degeneracies indetermining other cosmological parameters via CMB, SNIaand BAO surveys (see, e.g., Dalal et al. 2006). Even with thesecond-generation GW detector network, the modest THE-SEUS EM + GW triggered coincidence number of 3-4 pre-dicted within the nominal mission life (not accounting forpossible “off-axis” prompt GRB detection), can rises to 10or more when including SXI follow-up observations of GWnetwork error boxes. With 10 GW+EM events, the Hub-ble constant could be constrained to 2-3%, thus providinga precise independent measure of this fundamental param-eter (Dalal et al. 2006; Nissanke et al. 2010). In addition,each individual joint GW+EM observation would providean enormous science return from THESEUS. For example,the determination of the GW polarization ratio would con-strain the binary orbit inclination and hence, when combinedwith an EM signal, the jet geometry and source energetics.Likewise, a better understanding of the NS equation of statecan follow from combined GW and EM signals (see, e.g.,Bauswein and Janka 2012; Takami et al. 2014; Lasky et al.2014; Ciolfi and Siegel 2015b,a; Messenger et al. 2015; Rez-zolla and Takami 2016).

¿

NS-NS /NS-BH mergers: Optical/NIR and soft X-ray isotropicemissionsNearly isotropic EM emission is expected from NS-NS /

NS-BH mergers at minutes-days time scale from the merger.GW emission depends only weakly on the inclination angleof the inspiral orbit, and GW detectors will mostly triggeron off-axis mergers (i.e. for binary system with a non zeroinclination angle). Both serendipitous discoveries within thelarge THESEUS/SXI FoV and re-pointing of THESEUS inresponse to a GW trigger will allow to study off-axis X-ray emission. One expected EM component is the late after-glow from the laterally spreading jet as soon as it decelerates(“orphan afterglows”; van Eerten et al. 2010). Peak bright-ness is expected at 1-10 days after the trigger, with peakX-ray fluxes equal or below ∼10−12-10−13 erg cm−2 s−1 at∼200 Mpc (Kanner et al. 2012). Therefore, despite their lowcollimation, off-axis afterglows will be detected only for themost nearby CBC systems. Another nearly-isotropic emit-ting component is expected if a massive millisecond mag-netar is formed from two coalescing NSs. In this case, X-ray signals can be powered by the magnetar spindown emis-sion reprocessed by the matter surrounding the merger site(isotropically ejected during and after merger), with lumi-nosities in the range 1043-1048 erg s−1 and time scales ofminutes to days (Metzger and Piro 2014; Siegel and Ciolfi2016a,b). Alternatively, X-ray emission may come from di-rect dissipation of magnetar winds (see, e.g., Zhang 2013;Rezzolla and Kumar 2015). Numerical simulations suggestthat such emission is collimated but with large half-openingangles (30-40 deg, beaming factor of ∼0.2). As an additionalchannel in X-rays, the magnetar may accelerate the isotrop-ically expelled matter through wind pressure to relativisticspeeds generating a shock with ISM (“confined winds”).Synchrotron radiation produced in the shock is emitted nearlyisotropically, with an enhanced intensity near the equator. Abeaming factor of ∼0.8 is expected in this case (see, e.g.,Gao et al. 2013). Overall, typical time scales for these X-raysignals are comparable to magnetar spin down time scalesof ∼103-105 s, and the predicted luminosities span a widerange that goes from 1041 erg s−1 to 1048 erg s−1. With THE-SEUS/SXI in combination with the second-generation de-tector network, almost all X-ray counterparts of GW eventsfrom NS-NS merging systems will be easily detected simul-taneously with the GW trigger and/or with rapid follow-upof the GW-individuates sky region. These X-ray emissioncounterparts could be possibly detected up to large distanceswith the third-generation of GW detectors (depending on thelargely uncertain intrinsic luminosity of such X-ray compo-nent, see Table 1), providing a unique contribution to clas-sify X-ray emission from NS-NS systems and probes thecosmic evolution.

In the optical band, the expected EM component is rep-resented by the so-called “macronova” (often named “kilo-

THESEUS 13

Table 1 Number of NS-NS (BNS) mergers expected to be detected in the next years by second- (2020+) and third- (2030+) generation GWdetectors and the expected number of electromagnetic counterparts as short GRBs (collimated) and X-ray isotropic emitting counterparts (see,e.g., Ciolfi and Siegel 2015b; Rezzolla and Kumar 2015) with THESEUS SXI and XGIS. BNS horizon indicates the GW detector sensitivity (see,e.g., Abadie et al. 2010a). The rate estimates of simultaneous GW+GRB detections assume that all BNS can produce a short GRB and take intoaccount a combination of collimation angle range, XGIS FoV as a function of energy, and possible prompt off-axis detection. X-ray counterpartrate estimates assume that at least 1/3 of BNSs produce a long-lived NS remnant (but see Gao et al. 2016; Piro et al. 2017).

GW observations THESEUS XGIS/SXI joint GW+EM observations

Epoch GW detector BNS horizon BNS rate XGIS/sGRB rate SXI/X-ray isotropic(yr−1) (yr−1) counterpart rate (yr−1)

2020+ Second-generation (advanced LIGO, ∼400 Mpc ∼40 ∼5-15 ∼1-3 (simultaneous)Advanced Virgo, India-LIGO, KAGRA) ∼6-18 (+follow-up)

2030+ Second + Third-generation ∼15-20 Gpc >10000 ∼15-25 &100(e.g. ET, Cosmic Explorer)

Fig. 12 Expected X-ray fluxes at peak luminosity from different modelling of X-ray emission from NS-NS merger systems, that are among themost probable GW sources that will be detected in the following years by the second- and third- generation GW detectors. Grey solid lines showa typical GRB X-ray afterglow observed with Swift/XRT. Dots show the expected flux using fiducial parameters for each model. The black anddashed line show the THESEUS/SXI sensitivity as a function of the exposure time (credit: S. Vinciguerra; see also Fig. 26)

nova”) emission (e.g., Li and Paczynski 1998). During NS-NS or NS-BH mergers, a certain amount of the typdally dis-rupted neutron star mass is expected to become unbound andejected into space. This matter has the unique conditions ofhigh neutron density and temperature to initiate r-processnucleosynthesis of very heavy elements. Days after merger,the radioactive decay of such elements heats up the ejectedmaterial producing a thermal transient signal peaking in theoptical/near-infrared (NIR) band and with luminosities of∼1040 erg s−1 (see, e.g., Fernandez et al. 2016; Baiotti andRezzolla 2017). Interaction of the ejected matter with sur-rounding ISM may also produce synchrotron radiation atlate times (∼weeks-months) peaking at radio wavelengths.Another fraction of the tydally disrupted neutron star mat-ter remains bound to the system forming an accreting disc.Disc winds outflows are expected to produce a more blueoptical emission peaking at earlier epochs (e.g. 1-2 days af-ter the merger), with luminosity of the order of a few times1040−41 erg s−1, depending also on the nature of the centralremnant (Kasen et al. 2015).

Macronovae are promising electromagnetic counterpartsof binary mergers because (i) the emission is nearly isotropicand therefore the number of observable mergers is not lim-ited by beaming; (ii) the week-long emission period allowsfor sufficient time needed by follow-up observations. Thedetectability of macronovae is currently limited by the lackof sufficiently sensitive survey instruments in the optical/NIRband that can provide coverage over tens of square degrees,the typical area within which GW events will be localizedby the Advanced LIGO-Virgo network. So far, observationalevidence of the macronova emission was obtained only infew cases during the follow-up campaigns of the optical af-terglows of short GRBs (Tanvir et al. 2013; Jin et al. 2015).Figure 13 shows the expected macronova apparent magni-tudes for a source at 200 Mpc as well as the expected intrin-sic luminosity (Metzger and Berger 2012). Once the macronovacomponent is identified, source location can be accuratelyrecovered, allowing for the identification of the host galaxyand the search for counterparts in other electromagnetic bands(e.g. radio).

14 Amati et al.

Fig. 13 The dark grey region shows the expected macronova r-bandapparent magnitude for a source at 200 Mpc as a function of timefrom the burst onset. Solid curves show the expected GRB afterglowemission assuming different energetics and ISM densities. Red squaresand blue triangles represent the afterglow detection (squares) and up-per limits (triangles) for a sample of short GRBs (Metzger and Berger2012).

During the next decade (2020-2030) we expect strongsynergies between the second-generation GW detector net-work and several telescopes operating at different wavelegthssuch as: 1) the space-based telescopes James Webb SpaceTelescope (JWST), ATHENA and WFIRST; 2) the ground-based telescope with large FOV like zPTF and LSST, whichwill be able to select the GW candidates in order to follow-up them afterwards; 3) the large telescopes, such as the 30-mclass telescopes GMT, TMT and ELT, which will all follow-up the optical/NIR counterparts like macronovae; 4) the SquareKilometer Array (SKA) in the radio, which is well suited todetect the late-time (∼weeks) signals produced by the in-teraction of the ejected matter with the interstellar medium.THESEUS/IRT will be perfectly integrated in this search,due to its photometric and spectroscopic capabilities and itsspectral range coverage. Light curves and spectra will be ac-quired, thus giving the opportunity to disentangle among ex-pected different components associated to macronova events(e.g. disk wind + dynamical ejecta contributions; Kasen et al.2015).

Core collapse of massive stars: Long GRBs, Low Luminos-ity GRBs and SupernovaeThe collapse of massive stars are expected to emit GWssince a certain degree of asymmetry in the explosion is in-evitably present. Estimates of the GW amplitudes are stillpoorely constrained and the expected output in energy hasenormous uncertainties, ranging from 10−8 to 10−2 Moc2. Ifthe efficiency of the GW emission is effectively very low,then only the third-generation GW detectors such as ET,which will be operative at the same time as THESEUS, willbe able to reveal the GW emission from these sources and

thus obtain crucial insights on the innermost mass distribu-tion, inaccessible via EM observations. The collapsar sce-nario invoked for long GRBs requires a rapidly rotating stel-lar core (see, e.g., Woosley 1993; Paczynski 1998), so thatthe disk is centrifugally supported and able to supply thejet. This rapid rotation may lead to non-axisymmetric in-stabilities, such as the fragmentation of the collapsing coreor the development of clumps in the accretion disk (Gia-comazzo et al. 2011). GW/EM synergy plays a crucial rolein the investigation of the nature of GRB progenitors andphysical mechanisms. Indeed, gamma-ray emission and themultiwavelengths afterglow are thought to be produced atlarge distances from the central engine (i.e. >1013 cm). Bycontrast, GWs will be produced in the immediate vicinityof the central engine, offering a direct probe of its physics.Inspiral-like GW signals are predicted with an amplitudethat might be observable by ET to luminosity distances oforder 1 Gpc (see, e.g., Davies et al. 2002). With a rate ofobserved long GRB of ∼0.5 Gpc−3 yr−1, THESEUS couldprovide a simultaneous EM monitoring of ∼1 long GRBsevery two years. Off-axis X-ray afterglow detections (“or-phan afterglows”) can potentially increase the simultaneousGW+EM detection rate by a factor that strongly depends onthe jet opening angle and the observer viewing angle. THE-SEUS may also observe the appearance of a NIR orphanafterglow few days after the reception of a GW signal dueto a collapsing massive star. In addition, the possible largenumber of low luminosity GRBs (LLGRBs) in the nearbyUniverse, expected to be up to 1000 times more numerousthan long GRBs, will provide clear signatures in the GWdetectors because of their much smaller distances with re-spect to long GRBs. GW+EM synergy plays a crucial rolein the investigation of the nature of GRB progenitors andpysical mechanisms. Indeed, gamma-ray emission and themultiwavelengths afterglow are thought to be produced atlarge distances from the central engine (i.e., > 1013 cm). Bycontrast, gravitational waves will be produced in the innerregions of the central engine, offering a direct probe of itsphysics.

Soft Gamma RepeatersFractures of the solid-crust on the surface of highly mag-netized neutrons stars and dramatic magnetic-field readjust-ments represent the most widely accepted explanation to in-terpret X-ray sources such as giant flares and soft gammarepeaters (SGRs; see, e.g., Thompson and Duncan 1995).NS crust fractures have also been suggested to excite nonradial oscillation modes that may produce detectable GWs(see, e.g., Corsi and Owen 2011; Ciolfi et al. 2011). Themost recent estimates for the energy reservoir available ina giant flare are between 1045 erg (about the same as thetotal EM emission) and 1047 erg. The efficiency of conver-sion of this energy to GWs has been estimated in a number

THESEUS 15

of recent numerical simulations and has been found to belikely too small to be within the sensitivity range of presentGW detectors (Ciolfi and Rezzolla 2012; Lasky et al. 2012).However, at the typical frequencies of f-mode oscillationsin NSs frequencies, ET will be sensitive to GW emissionsas low as 1042-1044 erg at 0.8 kpc, or about 0.01% to 1% ofthe energy content in the EM emission in a giant flare. In theregion of 20-100 Hz, ET will be able to probe emissions aslow as 1039 erg, i.e. as little as 10−7 of the total energy bud-get (see, e.g., Chassande-Mottin et al. 2010, and referencetherein).

Neutrino sourcesSeveral high-energy sources that THESEUS will monitorare also thought to be strong neutrino emitters, in particu-lar SNe and GRBs. The shocks formed in the GRB ultra-relativistic jets are expected to accelerate protons to ultrarel-ativistic energies and that, after interacting with high energyphotons, produce charged pions decaying as high energyneutrinos (>105 GeV; see, e.g., Waxman and Bahcall 1997).Pulses of low energy neutrinos (<10 MeV) are expected tobe released during core-collapse supernovae (CCSNe) withan energy release up to 1053 erg. Indeed, low energy neutri-nos have been detected from SN1987A at 50 kpc distance.Still significant uncertainties are affecting supernova mod-els. GW and neutrino emission provide important informa-tion from the innermost regions as the degree of asymmetryin the matter distribution, as well the rotation rate and thestrength of the magnetic fields, that can be used as priorsin numerical simulations (see, e.g., Chassande-Mottin et al.2010, and reference therein). Because of the neutrino verysmall cross-sections and low fluxes, neutrino detectors nec-essarily require huge amounts of water or liquid scintillator.Future Megatons detectors that are expected to work dur-ing the 3rd generation GW detectors, will reach distances upto 8 Mpc, that would guarantee simultaneous GW/neutrinoand EM detection of ∼1 SN per year. Very promising forsuch multi-messenger studies are the LLGRBs, given theirexpected larger rate than for standard long GRBs (up to 1000times more numerous) and their proximity. For long GRBs,joint THESEUS and GW/neutrino observations would fur-ther constrain progenitor models, clarifying the fraction ofenergy channeled via dynamical instabilities (Fryer et al.2002) and the relative neutrino/EM energy budgets. Neu-trino observations would also constrain the composition ofthe GRB jet and the relation of GRBs to high-energy cosmicrays (Abbasi et al. 2012).

2.3 Exploring the Time-domain Universe

The SXI and XGIS will detect a large number of both tran-sient (Fig. 15) and steady X-ray sources serendipitously dur-ing regular observations (see Table 2). These data will pro-

vide a wealth of science opportunities. Here we emphasisetwo primary objectives:

– reveal the violent Universe as it occurs in real-time, throughan all-sky X-ray survey of extraordinary grasp and sen-sitivity carried out at high cadence.

– discover new high-energy transient sources over the wholesky, including supernova shock break-outs, black holetidal disruption events, magnetar flares, and monitor knownX-ray sources, including GRBs, with low latency obser-vations.

These objectives relate directly to the Cosmic Vision 2015-2025 questions under (3.2), providing the crucial electro-magnetic counterparts to GW sources such as compact starbinary in-spirals and perhaps core-collapse SN, under (3.3),allowing the study of matter under extreme conditions aroundblack holes and in neutron stars, and under (4.3), examiningthe evolving violent Universe through the study of quiescentand active massive black holes at the centers of galaxies.By finding huge numbers of GRBs the survey will also per-mit unprecedented insights in the physics and progenitors ofGRBs and their connection with peculiar core-collapse SNe,and substantially increase the detection rate and characteri-zation of sub-energetic GRBs and X-Ray Flashes. The pro-vision of a high cadence soft X-ray survey in the 2020s to-gether with a 0.7 m IRT in orbit will enable a strong synergywith transient phenomena observed with the Large SynopticSurvey Telescope (LSST).

SupernovaeSupernovae mark the death of massive stars and are the primeprocess by which heavy elements are distributed through theUniverse driving evolution in the stellar population. THE-SEUS will significantly advance our understanding of theSN explosion mechanism, detecting SNe at the very mo-ment of emergence, gathering comprehensive, prompt dataand alerting follow-up communities to new events. The birthof a new SN is revealed by a burst of high-energy emissionas the shock breaks out of the star (giving access to measure-ments of the progenitor star radius). This has been spectacu-larly captured just once, in a serendipitous Swift XRT obser-vation of SN2008D (Soderberg et al. 2008): SNe are usuallyfound only days to weeks after the explosion, as radioac-tive heating powers optical brightening. Theoretical calcu-lations of shock breakout show that bright X-ray bursts ofLX = 1043 − 1046 erg s−1 are expected for both Wolf-Rayetstars and red supergiants (RSG) lasting 10-1000 s (Nakarand Sari 2010). These progenitors are likely responsible forType Ibc and most Type II SN respectively, which occur atrates of 2.6×10−5 and 4.5×10−5 Mpc−3 yr−1 (Li et al. 2011).

SXI can detect these landmark events out to galaxies be-yond 50 Mpc. These bursts first emit hard spectra when theshock is relativistic (Nakar and Sari 2012), also allowing de-tection by the XGIS. Shock breakouts from blue supergiants

16 Amati et al.

Fig. 14 Possible 1-year THESEUS/SXI sky exposure map (Galactic coordinates) based on the observing constraints and strategy described in nextsections.

Fig. 15 Left: Time scales and luminosities of presently known soft X-ray transients (Jonker et al. 2013). Right: typical light curve of an X-RayFlash, i.e. a GRB showing emission only in the X-ray energy range (BeppoSAX XRF020427; Amati et al. 2004).

Table 2 THESEUS detection rates for different astrophysical tran-sients and variables.

Transient type SXI rate

Magnetars 40 day−1

SN Ia shock breakout 4 yr−1

TDE 50 yr−1

AGN+Blazars 350 yr−1

Thermonuclear bursts 35 day−1

Novae 250 yr−1

Dwarf novae 30 day−1

SFXTs 1000 yr−1

Stellar flares and super flares 400 yr−1

are expected to release comparable energy to RSGs, emit-ting within the XGIS band.

With THESEUS, we aim at finding the first X-ray burstsfrom thermonuclear Type Ia SN shock breakouts. The detec-tion of even a single Ia X-ray event is a powerful discrimi-nator between progenitor models, and constrains the explo-

sion physics with implications for using SN Ia as standardcandles and to constrain dark energy (Sullivan et al. 2011).Galactic Ia shock breakouts should be detectable by XGIS asvery short high energy pulses (Nakar and Sari 2010), whilethose in binary with a red giant star may produce bright X-ray bursts lasting minutes to hours via interaction with thecompanion wind (Kasen 2010).

Relativistic SN shock breakout can also contribute a sig-nificant fraction of the early X-ray flux seen in low-luminosityGRBs (see the cases of GRB 060218, and GRB 100316D;Campana et al. 2006; Starling et al. 2011), and THESEUScan make a significant contribution to this study, bridgingthe temporal gap between high and low energy X-rays ofcurrent instrumentation. Fundamental parameters and pro-cesses, such as the radius of breakout and the driver of thelight curve time scale, as well as the nature of the progenitorstars themselves, remain unknown. THESEUS will discoverall types of SN shock breakouts opening up this critical andunexplored time domain in SN evolution.

THESEUS 17

Tidal Disruption EventsTidal disruption events (TDEs) offer a unique probe of theubiquity of super-massive black holes (SMBHs) in galaxies,accretion on timescales open to direct study, and the natureand dynamics of galactic nuclei. A star is tidally disruptedwhen the tidal forces from the SMBH exceed the self-gravityof the star (Rees 1988). At this point half of the star is un-bound, while the other half falls back to form an accretiondisc at a characteristic rate of t−5/3. Such events are expectedto be visible in UV and soft X-rays (see, e.g., Komossa et al.2004; Gezari et al. 2012). The discovery by Swift of twohighly luminous outbursts from galactic nuclei implies thatat times a fraction of this energy is deposited in a new rel-ativistic jet outflow (Levan et al. 2011; Bloom et al. 2011;Burrows et al. 2011), offering a new route to their identifica-tion and an opportunity to study newly-born jets. THESEUSis ideal for both the discovery and characterization of TDEs,opening new windows on numerous astrophysical questions.

TDEs offer a unique probe of SMBH presence across theUniverse by revealing formerly dormant SMBH in galaxiesacross the mass scale up to ∼108 M. The key to extractingthe most from these events is to obtain identifications early,while the first material is falling back. This in turn allowsmulti-wavelength observations at the peak of the light fromwhich one can infer the mass of the black hole and the natureof the in-falling star. This can be compared to the propertiesof the galaxy and the MBH-σ relation. TDEs provide uniquelaboratories for studies of physics under extreme conditions.They allow us to study accretion in active galaxies from on-set to a return to dormancy, on timescales of only a fewyears. Through this time we can study the behaviour of ac-cretion at a variety of rates, and the processes of disc and jetformation. Jets in the relativistic TDEs are promising loca-tions for the acceleration of ultra-high-energy cosmic-rays.Finally, the rate and nature of the TDEs provides insight intothe central dynamics of galaxies. An exciting possibility isusing TDEs as tracers of BH-mergers. The dynamical im-pact of the merger on surrounding stars increases the TDErate by several orders of magnitude to > 0.1 yr−1, raising thepossibility of observing multiple events from a single galaxyundergoing a BH merger. For the classical TDEs, the SXI ef-fective horizon in a single orbit is ∼200 Mpc (z ∼0.05). Therate of relativistic TDEs is uncertain, but both SXI and XGIScan see them to z ∼1 (Lpeak = 1048 erg s−1). For moderatelyconservative assumptions (beaming to 5% of the sky in 10%of TDEs) it is quite possible that the relativistic TDE ratewill exceed the classical one.

AGN and BlazarsActive galaxies are the most powerful continuous sourcesof energy in the Universe and are powered by supermassiveBHs in galactic centres. They also help control the growth ofthe stellar population in galaxies, so understanding their ac-

cretion activity is crucial. The accretion activity of the cen-tral BH in active galaxies varies over a large range in am-plitude and timescale, due to variations in the emission effi-ciency or accretion rate/efficiency in AGN (see, e.g., Mushotzkyet al. 1993), or to changes in relativistic jet properties inBlazars (see, e.g., Marscher et al. 2010). SXI provides thecapability to monitor the X-ray flux of hundreds of AGNwith ∼10% accuracy on daily timescales, and hundreds moreon longer timescales (Ueda et al. 2005). The survey strat-egy will permit an unbiased look at the long-term variabilityof an unprecedentedly large AGN/Blazar sample at depthsnever reached before. SXI monitoring will enable regularmulti-wavelength campaigns to occur in order to probe theemission mechanisms and the geometry of the central activeregions.

The observation of correlated X-ray/radio AGN variabil-ity with THESEUS and SKA, can be strongly diagnostic, al-lowing searches for mass-related lags and radio-frequency-related lags, opening up the jet physics. It is generally agreedthat the radio to UV/X-ray emission from blazars is syn-chrotron emission from a relativistic jet oriented towardsthe observer, with the higher energy emission arising fromCompton scattering of seed photons by particles in the jet,although many details of the jet structure are still unclear.The expected sensitivity of future radio observations is suchthat radio-X-ray studies of radio quiet Seyfert galaxies willthen be possible. In particular, THESEUS will allow the de-tection of X-ray flaring activity in Seyferts. Connecting flarestates and X-ray/radio variability in radio quiet AGN, wouldhave implications for AGN evolution/feedback models.

THESEUS will monitor the bright AGN population andtrigger follow-up observations by both THESEUS itself andother multi-wavelength facilities to measure the relation be-tween different energy bands and the lag between bands whichconstrain the emission process. For example, SynchrotronSelf-Compton emission predicts a TeV lag roughly equalto the light travel time across the emission region, whereasfor external seed photons, assuming the source of variabil-ity is in the relativistic electrons of the jet, simultaneity isexpected. Measurement of the complete shape of the syn-chrotron spectral component from radio to hard X-ray con-strains the particle acceleration process in the jet and thebalance of acceleration and radiative cooling; if energy isinput via hadrons rather than via electrons, radiative losseswill be less, so the spectrum will be harder at higher energies(see, e.g., Tramacere et al. 2007). In the THESEUS era, tentimes greater TeV sensitivity than now will be provided bythe Cherenkov Telescope Array, allowing the routine studyof hundreds of blazars. VHE gamma ray emission is char-acterized by large outbursts during which detailed measure-ments can be made. Deep monitoring observations can alsobe made with XGIS whose hard X-ray spectral response isideal for defining the synchrotron shape.

18 Amati et al.

Accreting BinariesThermonuclear X-ray bursts are produced by runaway nu-clear burning on NS surfaces in our galaxy, often reach-ing the Eddington luminosity limit (Strohmayer and Watts2006). THESEUS probes a regime of deep nuclear carbonburning, and its thermal effects on neutron star crusts andcores, rarely observed before now (Brown 2004; Cummingand Macbeth 2004). These infrequent hours-long “super-bursts” are efficiently detected in the THESEUS sky sur-vey due to its high exposure. It also captures seconds-longhydrogen- and helium-burning thermonuclear bursts morefrequently. A large sample of burst properties will test mod-els that predict nuclear burning dependence on mass accre-tion rate (Heger et al. 2007), and add to previous burst sam-ples collected by BeppoSAX and RXTE (Keek et al. 2010).

Classical and Recurrent Novae are produced by runawaythermonuclear burning on the surfaces of a white dwarf in aclose binary system. SXI will, for the first time, detect theinitial thermonuclear runaway burning phase, which lastsonly a few hundred seconds. Previous X-ray studies haveonly been able to probe the shock-heated wind ahead ofthe ejecta and the steady burning phase that emerges later(see, e.g., Osborne et al. 2011; Schwarz et al. 2011; Os-borne 2015). X-ray spectra and temporal profiles provideconstraints on nuclear reaction processes, the white dwarfmass, and the underlying convective mixing (see, e.g., Star-rfield et al. 2009). THESEUS will be able to monitor thebrightnesses of the Super-Soft Sources (Greiner 1996), can-didate progenitors for Type Ia SNe, to give a significantlyimproved view of their accretion behaviour. Both Classicaland Recurrent Novae are also sources of hard X-ray emis-sion (Sokoloski et al. 2006; Mukai et al. 2008) variable withtime which appears later than the SSS phase. The onset ofthis emission component probes the existence of shockedmaterial within the ejected shell. In addition the recent andnew discoveries of Novae (both classical and recurrent) assource of particle acceleration by Fermi-LAT, and thus aclass of gamma-ray emitters, challenges our knowledge ofthese transient objects (Cheung et al. 2016). The wide en-ergy coverage of the SXI and XGIS instruments will al-low us to track the temporal evolution of the different emis-sion components along the outburst. All this will provide asignificant improvement of our still poor knowledge of thetight link between accretion processes and explosion mech-anisms.

Accretion-driven outbursts and state changes are alsoseen in white dwarf, NS and BH binaries. Dwarf Novaeoutbursts occurring in white dwarf accreting systems (cat-aclysmic Variables) will be routinely observed in both softX-rays and optical/nIR ranges. This unique opportunity willallow to solve the still open question on dwarf nova diversityin optical and X-ray behaviours (Fertig et al. 2011). Further-more, cataclysmic variables were thought to be unable to

launch jets. The recent radio discovery of two high accretionrate systems, including dwarf novae (Kording et al. 2008,2011), makes crucial the sinergy with SKA to understandjet-launching processes irrespective of the compact objectnature (WD, NS or BH). Truly magnetic systems are theX-ray brighest among CVs. However the state changes arenot abrupt but on a long-term scale (months-yrs). THESEUSwill provide excellent coverage of the outbursts of classicalneutron star and black hole X-ray binaries, for example Su-pergiant Fast X-ray Transient variability reaches up to a fac-tor of 106 (Romano et al. 2015), with peak luminosities upto 1038 erg s−1; the hour-timescale flares from these OB plus(presumed) NS systems have frequently triggered the Swift-BAT. THESEUS can constrain the temperature and opticaldepth of the accretion column (Farinelli et al. 2012; Bozzoet al. 2016), and the origin of the bright flares possibly dueto wind accretion onto a magnetar (Bozzo et al. 2008). THE-SEUS will detect and provide localization for several black-hole transients, monitoring daily their X-ray spectral evo-lution throughout their full outburst. Pointed observationswith IRT will provide strictly simultaneous IR photometry(and often spectroscopy) with very good statistics, allowingan unprecedented study of the disk-jet connection across allaccretion regimes.

MagnetarsMagnetars, young NS with external magnetic fields of 1013-1015 G, are among the most powerful and spectacular high-energy transients in the sky. They are characterized by theemission of highly super-Eddington short bursts of emission(Soft Gamma-ray Repeaters) and more rare Giant Flares (lu-minosity up to 1047 erg s−1). Magnetars differ from othermore common classes of neutron stars because all their emis-sion (both persistent and bursting) is powered by the gradualand/or impulsive dissipation of magnetic energy, rather thanby rotational energy or accretion (Thompson and Duncan1995). There is evidence that the field in the interior of mag-netars can exceed 1016 G, probably as a result of their rapidrotation at birth (1-2 ms). About twenty sources believed tobe magnetars are currently known in our Galaxy and in theMagellanic Clouds, but since most of them are transientswith long quiescent periods, the total population waiting tobe discovered is certainly much larger (Mereghetti 2008).

Magnetars can produce Giant Flares thought to be due tostar crustal fractures. Their EM emission consists of an ini-tial, short (<0.5 s) spike of hard X-rays followed by a tail ofsofter X-rays lasting minutes and modulated by the NS ro-tation (P∼2-12 s). These extremely bright initial spikes canbe detected with the XGIS to considerable distance. Basedon the rate of the few Giant Flares observed to date, and theXGIS’s energy range will be better suited for the detectionof such events than current coded-mask detectors due to itslower energy threshold.

THESEUS 19

The persistent X-ray emission from known magnetars istoo faint and strongly absorbed to be detectable by SXI whenthese sources are in a quiescent state; however, the increasedlevel of X-ray emission associated with flares and periodsof increased bursting activity will be easily detectable. ThusTHESEUS triggers will enable detailed observations of theseevents. These “intermediate flares” are more frequent thanthe Giant Flares. The wide field of view of SXI and the fre-quent sky coverage will, for the first time, allow detection ofa large number of flares and obtain a reliable estimate of thefrequency of such events. The count rate expected in SXI fora typical intermediate flare will allow detailed time-resolvedstudy of flare properties.

Stellar CoronaeStellar Flares of all sizes are important probes of coronalstructures and their energetics, and thus, the underlying stel-lar magnetic dynamo (see, e.g., Reale 2007; Aschwandenand Tsiklauri 2009). THESEUS will quantify the extreme“super-flares” of nearby magnetically active stars>104 timesmore energetic than the Sun’s largest flares. X-ray flareshave long been detected and cataloged (see, e.g., Pye andMcHardy 1983; Favata 2002; Gudel 2004), but rare “super-flares” can briefly (∼1000 s) exceed the star’s quiescent bolo-metric luminosity, and only a handful are known (see, e.g.,RS CVn binary II Peg and dMe star EV Lac; Osten et al.2007, 2010). As an X-ray mission, THESEUS will directlyaddress the high-energy ionising radiations of primary im-portance for solar-terrestrial interactions. THESEUS will pro-vide estimates of coronal loop structure sizes and energy re-lease in flares. Occurrence rates and luminosities constrainstellar models, and also affect the conditions for life in thehabitable zone. Although much progress has been and con-tinues to be made from studies of our own Sun, many ques-tions remain regarding fundamental flare processes (see, e.g.,Gudel 2004; Benz and Gudel 2010); THESEUS will allowthe theoretical models for energy release and propagation tobe tested within a greatly expanded phase space. SXI willbe the primary instrument for the stellar-flare survey, andits photon-energy bandpass is well-tuned to stellar-coronalemissions (which have typical characteristic temperatures∼0.1-10 keV).

2.4 GRB physics, progenitors and cosmology

X-ray Flashes, sub-energetic GRBs and GRB-SN connectionIt is well established that long GRBs are linked to SN origi-nating from the core collapse of a stripped-envelope massivestar (SN type Ibc). However, for GRBs the bulk of the ex-plosion energy goes into relativistic ejecta, while the vastmajority of SN Ibc show sub-relativistic ejecta with an en-ergy release several orders of magnitude lower than GRBs.

Some SN Ibc must therefore harbour an additional key in-gredient, i.e. a central engine - likely a nascent black hole- that drives the explosion and launches a relativistic out-flow producing the GRB. Why this happens is not clear. Themissing link between the two classes of explosions may befound amongst the X-Ray Flashes (XRFs, see Fig. 16), com-monly considered as a softer sub-class of GRBs (but seealso Ciolfi (2016)), and LL-GRBs, likely related to a pop-ulation of only slightly relativistic SN which have recentlybeen found in the radio. Current X-ray facilities are quite in-sensitive to such events, which are typically characterized bysoft spectra (low Epeak). These events will make up ∼1/3 ofthe THESEUS GRBs, populating the existing gap betweenGRBs and ordinary SN (Fig. 15). Because of their low lumi-nosity, we expect that most of these events will be at z <0.5and that, in terms of rate density, they constitute the bulk ofGRB population. Present surveys are in fact biased towardsharder, more luminous events (less than 5% of long Swiftbursts are at z <0.5; Jakobsson et al. 2012). The detection ofintrinsically faint, X-ray soft GRBs can only be done at lowX-ray energies, when there are enough photons for their de-tection. An important goal of THESEUS is to understand thepaths of stellar evolution leading to the production of GRBsand of SNe, as described in below.

GRB physics and circum-burst environmentCurrent GRB facilities do not permit prompt X-ray obser-vations in the spectral band where most of the photons areemitted. Around 60% of THESEUS GRBs will be simulta-neously detected by SXI and XGIS during the prompt emis-sion, allowing for the first time measures of the energy spec-trum from 0.3 keV to 10 MeV, in a domain little explored,but crucial to discriminate among different emission modelsand determine the effects of intrinsic absorption in the GRBenvironment (Fig. 17). Such observations are needed to val-idate models based on synchrotron emission and to measurethe impact of the GRB on its surrounding.

It is worth noting that a number of recent studies havefound evidence of a sub-dominant thermal emission in theprompt emission spectrum (Ryde et al. 2010; Guiriec et al.2011; Basak and Rao 2015). The thermal emission is alsoreported until very late emission phase of the longest ultra-long GRB 130925A and it has been shown to have a verysimilar spectral evolution as a ordinary long GRB 090618(Basak and Rao 2015). The former was observed with thefine resolution data of Swift/XRT, Nustar and Chandra, whilefor the later XRT data was available at the late prompt emis-sion phase. As such studies rely on the focusing observa-tions, they are rare and miss the evolution in the initial phaseof emission. With THESEUS such studies will be routinelydone and a broadband, fine resolution spectrum will be ob-tained from very early emission phase. Firstly, it will be anenormous step forward to study the evolution of the indi-

20 Amati et al.

Fig. 16 Left: distributions in the spectral peak energy (Ep) - Fluence plane of soft X-Ray Rich (XRR) GRBs and X-Ray Flashes (XRFs) comparedto that of normal GRBs (Sakamoto et al. 2005). Right: Blast wave velocity and energy for massive star explosions (adapted from Soderberg et al.2010). Soft/weak GRBs may constitute the bulk of the GRB population and the link with other explosive events associated to the death of massivestars.

vidual spectral components from the early phase, secondly,this will also increase the sample size and help in finding thespectral diversity or a unification across the GRB catalog.

GRB physics in EM-GWGRBs and, more generally, core-collapse supernovae maybe multi-messenger sources of electromagnetic (EM) andgravitational waves (GW) by their potential association withneutron stars and black holes. These alternatives may leavetheir imprint in high-frequency modulations of associatedhigh energy emission, in particular, by mis-alignment (align-ment) of magnetic moments with the spin of a magnetar(black hole).

High resolution light curves of relatively bright GRBsare hereby expected to show non-smooth (smooth) broad-band Kolmogorov spectra. Smooth broadband Kolmogorovspectra have been found up to the comoving frequency of afew kHz of bright LGRBs sampled at 2 kHz from the Bep-poSAX catalog (see Fig. 18).

THESEUS exceeds BeppoSAX in collecting area andtime resolution, allowing a more detailed analysis of individ-ual events and/or events at higher redshifts. Smooth broad-band spectra from THESEUS would further evidence theblack hole inner engines, absent any high frequency mod-ulations in spectra of gamma-ray light curves.

Results such as these provide potentially powerful pri-ors to our searches for GW emission accompanying LGRBs.LGRBs from rotating black holes, for instance, will pro-duce their highest GW frequencies from non-axisymmetricmass motion at the Inner Most Stable Circular Orbit (ISCO)powered by the ample reservoir in angular momentum of

a stellar mass black hole. Efficient extraction of broadbandspectra from gamma-ray light curves is made possible byGraphics Processor Units (GPUs), allowing deep searchesby using banks of millions of chirp templates developed toextract chirp-based spectrograms from noisy time-series ofGRBs (BeppoSAX, THESEUS) and LIGO-Virgo or KA-GRA alike.

Complete samplesIn order to study properties of GRBs, their X-ray and op-tical/NIR afterglows, and their hosts is important to handlea sample that is not biased towards classes (i.e. a completesample), because of limitation during the observations. Un-til now, several GRB complete samples have being created(see, e.g., Perley et al. 2016; Greiner et al. 2001; Salvaterraet al. 2012). In addition to these, other tools to overcome theproblems of unbiased distributions with robust and sophys-ticated statistical techniques have already been successfullyapplied to GRB prompt and afterglow emission (Dainottiet al. 2013a, 2015). The capability of THESEUS to detectmost afterglows in the IR, excluding the few highly extin-guished or at extreme redshift (<10%), will allow us for thefirst time to build a complete sample of GRB afterglows ob-served in X-rays and IR. The fact that THESEUS will not belimited by weather conditions and visibility constraints, butonly from pointing limitation and froregronud Galactic ex-tinction, is a strong advantage in respect to ground based fa-cilities dedicated to GRB follow-up (e.g. RATIR, GROND,REM).

THESEUS 21

Fig. 17 By satisfying the requirements for the main science (Fig. 22), in particular the extension of its energy band down to 2 keV with large areaand 300 eV energy resolution, the THESEUS/XGIS will show unique capabilities for discrmininating among different GRB emission models (left)and detecting absorption features expected at energies <10 keV (circum-burst environments and X-ray redshift determination). Left: SimulatedXGIS low-energy (SDD) spectra of the first 50 s of GRB 090618 (Izzo et al. 2012) obtained by assuming either the Band function (black) orthe power-law plus black-body model (red) which equally fit the Fermi/GBM measured spectrum, which is also shown (blue). The black-bodyplus power-law model components best-fitting the Fermi/GBM spectrum are also shown (black dashed lines). Right: Simulation of the transientabsorption feature in the X-ray energy band detected by BeppoSAX/WFC in the first 8 s of GRB 990705 (Amati et al. 2000) as would be measuredby the XGIS.

Fig. 18 Broadband spectrum obtained as an average of 42 relativelybright bursts from the BeppoSAX catalog through butterfly filtering(purple curve) using a bank of 8.64 million chirp templates. The broad-band Kolmogorov spectrum extends standard Fourier-based spectra(blue curve) up to a few kHz in the comoving frame. The absence ofany high frequency bump favors inner engines harboring black holesrather than magnetars (from van Putten et al. 2014).

Multiwavelength prompt emissionRight now, thanks to Swift/UVOT and ground based opti-cal/NIR telescopes dedicated to the rapid follow-up of GRBafterglows, it is possible to simultaneously follow-up theprompt emission from optical to X-ray to Gamma-rays. Theseobservations allow us not only to better constrain the spec-tral energy distribution from optical to Gamma-rays and theirlightcurves, testing the standard model, but also to test the

Fig. 19 The redshift distribution of THESEUS GRBs during a 5 yrmission lifetime compared to the actual distribution of Swift GRBs(blue) during the same period. GRBs with a photometric redshift are ingreen, and those with a spectroscopic redshift are in red.

nature of the central engine. Indeed, the observer may seesimultaneously photons that have been emitted in differenttimes and regions of the flow, and also with different phys-ical origin, e.g., synchrotron or synchrotron self-comptonemission. However, there are only a few tens of bursts in12 years of Swift activity that could be long and bright enoughto be detected in optical (Levan et al. 2014). Indeed, up tonow only five events have been studied in such detail (see,e.g., Bloom et al. 2009; Rossi et al. 2011; Stratta et al. 2013;Troja et al. 2017), and while they probe that standard fireball

22 Amati et al.

model can explain the observations, in some cases the opti-cal and high-energy emission seems unrelated, or require amore complex modeling of the jet structure. Moreover, theseobservations have been performed in the optical, which ismore affected by foreground and host line of sight dust ex-tinction. With THESEUS/IRT capability of starting to obtainthe first images within the first 5 min from the trigger, it willbe possible to detect optical prompt emission for the longestGRBs, roughly 10 to 20 GRBs per year. This will dramati-cally increase the number of events to study, and allowing usto statistically explore the parameter space of several mod-els, shedding light on the structure of the jet during its firstphases.

X-ray early and late afterglowOpposed to what was initially believed, X-ray afterglows donot have a simple power-law decay. On the contrary, the vastmajority shows a canonical behavior (Nousek et al. 2006):an initial rapid decay during the first few hundred seconds,a slow fading plateau phase that can persist even longer than104 s, and a final power-law decay, with a possible achro-matic break. While it is clear that the initial rapid decay cor-responds to the tail of the prompt emission, the plateau phaseis difficult to explain within the collapsar scenario, becauseit requires a long activity of the central engine. Alternatively,the necessary energy may come from the spin-down activityof a magnetar formed during the collapse (see, e.g., Zhangand Meszaros 2001; Dall’Osso et al. 2011; Rowlinson et al.2014; Rea et al. 2015). To complicate this view, X-ray flares(not visible in gamma-rays), probably due to late emission,indicate that the central engine is still active. The relativis-tic outflow of a GRB is likely collimated in a jet (Sari andPiran 1999). Since the jet slows down, at some point therelativistic open-angle becomes larger than the relativisticjet-opening angle. Thus the observer starts to see a deficit ofphotons compared to the case of isotropic emission, leadingto an achromatic break in the light curve. These breaks, to-gether with the knowledge of the redshift and the measureof the isotropic energy, allow us to estimate the jet-openingangle and the collimated energy (and used as cosmologi-cal indicator). Achromatic jet-breaks have been observed inoptical/IR, however they often do not coincide with breaksobserved in X-rays, indicating that our knowledge of the af-terglows emission is still limited, and questioning if the jet-angle and collimated energy that have been estimated areright. Even if from the theoretical side many explanations(energy injections, double jet, structured jets) have been pro-posed to interpret the afterglow emission from IR/opticalto X-rays, more multi-frequency observations are necessary(see, e.g., Willingale and Meszaros 2017). Compared to to-day, the larger number of THESEUS GRBs and the moresensitive spectra observed with XGIS will allow us to bet-ter understand the nature of the afterglow and of the central

engine of GRBs. The study of the optical/NIR and X-rayafterglows unveils the properties of the environment. It iswell known that the circumburst density profile influencesthe shape of the GRB light curves and spectra (see, e.g.,Racusin et al. 2009) distinguishing by ISM and wind en-vironments (see, e.g., Schulze et al. 2011). Moreover, dustand gas in the line of sight dim the optical/NIR and X-rayafterglows, respectively (see, e.g., Greiner et al. 2011). Theirsystematic study will unveil the properties of the environ-ment where GRBs explode. Unfortunately, up to today thishas been limited by the different time coverage of X-rayand optical/NIR observations and sensitivity to the late af-terglows. THESEUS will likely solve this problem thanksto the simultaneous observations of optical/NIR and X-rayafterglows.

Optical/IR afterglow detection with IRTAs shown by Kann et al. (2017), within the first hour allknown optical afteglows have R<22. A classical optical af-terglow has a spectral slope β∼1, which translates in a colorR-H∼1 mag (AB photometric system), thus within the firsthour all known afterglows have H<21. IRT will observe op-tical afterglows longer than 30 min within 1 hour from thetrigger, reaching HAB∼20.6. The optical/NIR imager GROND,reaching 1 mag fainter limits only, has been able to detect∼90% of all GRBs detected by Swift within 4 hours fromthe trigger (Greiner et al. 2011). Note that the host extinctionwill mostly have a negligible effect, with only a few cases(∼10%) with AV>0.5, which will noticibly dim (>1 mag) theobserved NIR afterglow when the redshift is z>4, and stillobtaining a detection rate of ∼90%. However, at these red-shift dusty environments are less common, because dust didnot have the time to accumulate in the star forming regions.Notably, the higher rate of THESEUS GRBs will allow us tobetter understand the shape of dust extinction curve at highredshift which is now unexplored, the presence of 2175Aabsorption in GRB SEDs in high redshift environments andto test different models for dust grains.

Short GRBsShort GRBs are the least understood class of GRBs. Our un-derstanding of the nature and range of progenitors of shortGRBs remains hampered by their small number, and THE-SEUS will contribute importantly to increasing the samplefor study. More fundamentally, the observations of THE-SEUS will be crucial for the interpretation of candidate sig-nals in advanced detectors of gravitational waves. As dis-cussed above, THESEUS/XGIS will find ∼20 SGRBs yr−1,most of which will be localized more precisely with SXIfollow-up. At least 1/3 of the short GRBs are followed by aperiod of soft X-ray emission lasting 10-100 s. This emis-sion carries an energy comparable and often larger than theinitial spike, and it will be easily detected by THESEUS.

THESEUS 23

Fig. 20 Observed R band lightcurves of long GRBs. Highligthedis the ultra long GRB 111209A and the extremely extinguishedGRB 130925A (bottom curve). Data is corrected for Galactic extinc-tion. Adapted from Kann et al. (2017). We also show in the figure withblue lines the THESEUS/IRT sensitivity (dashed line spectroscopy,solid line imaging).

THESEUS will extend the search of this intriguing featuredown to a factor of ∼10 below present upper limits, allowingalso for the search of “orphan extended tails” of SGRBs ifthey are not beamed (Bucciantini et al. 2012).

Probing expansion history of the universe and dark energywith GRBsIn a few dozen seconds, GRBs emit up to 1054 erg in terms ofan equivalent isotropically radiated energy Eiso, so they canbe observed to z∼10 and beyond. By using the spectral peakenergy - radiated energy (or luminosity) correlation Ep,i -Eiso (Amati et al. 2002; Yonetoku et al. 2004), the luminos-ity at the end of the plateau emission and its rest frame du-ration correlation (Dainotti et al. 2008) and the fundamentalplane relation, an extension of the previous one by addingthe peak luminosity in the prompt emission (Dainotti et al.2016) are robust correlations studied and discussed for manyyears. Through these correlations, it has been demonstratedthat GRBs offer a very promising tool to probe the expansionrate history of the universe beyond the current limit of z = 2(Type-Ia SNe and Baryonic Acoustic Oscillations from QSOabsorbers). Additionally, the Ep,i - Eiso correlation has beenshown to hold within the individual pulses of GRB promptemission which is a strong argument against any instrumen-tal selection bias, and conveniently increases the sample sizefor such studies (Basak and Rao 2013).

With the present data set of GRBs, cosmological pa-rameters consistent with the concordance cosmology can

already be derived (see, e.g., Ghirlanda et al. 2004; Am-ati et al. 2008; Dainotti et al. 2013b). Current (e.g., Swift,Fermi/GBM, Konus-WIND) and forthcoming GRB exper-iments (e.g., CALET/ GBM, SVOM, Lomonosov/ UFFO,eXTP/ WFM) will allow us to constrain ΩM and the darkenergy equation of state parameters w0 and wa, describingthe evolution of w according to w = w0 + wa(1 + z), with anaccuracy comparable to that currently obtained with TypeIa supernovae (Fig. 21). The order of magnitude improve-ment provided by THESEUS on the sample of GRBs, withmeasured redshifts and spectral parameters, will allow us tofurther refine the reliability of this method. This will offerthe unique opportunity to constrain the geometry, and there-fore the mass-energy content of the universe back to z∼5,thus even extending the investigations of EUCLID and ofthe next generation large scale structure surveys to the en-tire cosmic history.

Synergies with JWST and E-ELTRecently, it has been shown that GRB 111209A is linked toa SN with properties dissimilar to any known GRB-SN, witha spectrum more in accordance with Superluminous super-novae (SLSN; see, e.g., Greiner et al. 2015), and like themwas probably not powered by the standard collapsar centralengine. The high rate of detection of THESEUS GRBs willsignificantly increase the number of long GRBs linked toSLSNe which can be studied with the planned JWST and E-ELT, shedding light on the GRB progenitors and their driv-ing mechanisms. With the exception of the brightest hostsat low redshift (z<2), IRT will not allow to detect hosts inNIR. However, their contribution will be visible as emissionand absorption lines in the afterglow spectra (Hjorth et al.2012; Savaglio et al. 2009), and therefore we will be able tomeasure their spectroscopic redshift. Moreover, the localiza-tion of the NIR afterglow and its immediate distribution tothe astronomical community, will permit others to follow-up the event with other ground and space based facilities,in particular the future instruments on the JWST and 30 mground based telescope (e.g., ELT, GMT, TMT). We thussummarize the most relevant synergies in the following twopoints:

– The Environment of GRBs with the future JWST and30 m class telescopes. THESEUS will enhance the rateof GRBs discovered at high redshift. At present the sam-ple of GRB host galaxies at z>6 is limited to 9 events.In the late 20’s the rate of GRBs at z>6 is expected to be>5 per year. Considering visibility constraints, we ex-pect to have ∼2-3 targets yr−1. JWST and future 30 mclass telescope will allow us to search even for dwarfhost galaxies with HAB ∼ 31 mag by using few hoursimaging with E-ELT. If the host will be brighter thanH∼27, spectroscopy will be possible. A prompt alert willpermit to use the optical afterglow as a reference star

24 Amati et al.

Fig. 21 Left: SNe-Ia + GRB Hubble diagram obtained by exploiting the correlation between spectral peak energy (Ep) and radiated energy orluminosity (Eiso, Liso) in GRBs (Demianski et al. 2017). Right: determination of the dark energy equation of state parameters w0 and wa with theexpected sample of GRBs form THESEUS by using the same (Amati and Della Valle 2013a).

Fig. 22 Conversion from THESEUS science goals to instrument and spacecraft requirements.

for Adaptive Optics (AO) in 30 m class telescope andperform IFU studies of the environment of the GRB,with angular resolution six times better than with JWST.This will provide information on the metallicity/dust/gasgradients close the GRB explosion sites, shedding lighton the most dusty environments (see, e.g., Rossi et al.

2012), on the aversion of long GRBs for high metallici-ties (see, e.g., Schulze et al. 2016), and in general on theproperties of the environment (e.g., star formation, gasinflow, dust content) needed for the formation of GRBprogenitors. One of the driving mechanisms for the for-mation of the massive stars at high redshift, including

THESEUS 25

progenitors of long GRBs, may be the inflow of pristinegas during galaxy interactions (see also Michałowski et al.2015). The high angular resolution and sensitivity achiev-able with imaging and spectroscopy instruments mountedon JWST and 30 m class telescopes will permit us tostudy the morphological properties and the UV emissionof the host of high-z GRBs. These observations will al-low us to test the hypothesis that galaxy interactions athigh redshift induce the formation of very massive starsand GRB progenitors.

– The missing host galaxies of short GRBs. The short GRBoffsets normalized by host-galaxy size are larger thanthose of long GRBs, core-collapse SNe, and Type IaSNe, with only 20% located at within the galaxy radius.These results are indicative of natal kicks or an origin inglobular clusters, both of which point to compact objectbinary mergers (Berger 2014). About 20% of well local-ized SGRBs (∼7% of the total number of SGRBs) havebeen classified as host-less (Fong et al. 2013). The non-optimal X-ray localizations do not permit us to solve thisproblem, because several galaxies lie within the X-rayerror circle, as it is shown in deep optical-NIR HST im-ages. THESEUS/IRT will allow us to localize sGRB af-terglows within a region (<1 arcsec error circle) smallenough to search for the host galaxies using the futureJWST and 30 m class telescopes and investigate if thesemight be inside very faint galaxies or may have beenejected by natal kicks or dynamics. Distinguishing be-tween these two scenarios gives constraints on the for-mation of NS binaries.

2.5 Observatory science with IRT

Fielding an IR-specified spectrograph in space, THESEUSwould provide a unique resource for understanding the evo-lution of large samples of obscured galaxies and AGN. Witha rapid slewing capability, and substantial mission duration,the mission will provide a very flexible opportunity for sev-eral fields of astrophysics, in a way similar to what is cur-rently done by Swift/XRT in the X-rays. For instance, itwill be possible to take efficient images and spectra of largesamples of galaxies with minute-to-many-hour-long cumu-lative integrations. A continuous spectral coverage with noblockages due to atmospheric opacity ensures that identicalspecies R lines can be tracked in extensive samples. The ca-pability to cover the redshift range from 0.07<z<1.74 for Hαand 0.44<z<2.29 for Hβ enables Balmer decrement mea-surements of the extensive evolution of the AGN and galaxyluminosity functions at redshift ∼0.5-1.5, a spectral regionthat simply cannot be covered from the ground. These keydiagnostic rest-optical emission lines will be observed forgalaxies in this substantial range of redshifts, reaching outtowards the peak of AGN and galaxy formation activity,

over a continuous redshift range where the bulge-blackholemass relation is being built up and established, and the mainsequence of star formation is well-studied. With excellentimage quality, THESEUS R∼500 grism can also providespatially-resolved spectral information to highlight AGN emis-sion, and identify galaxy asymmetries.

The imaging sensitivity of THESEUS is about 6 magni-tudes lower than for JWST in the same exposure time; nev-ertheless, its availability ensures that many important statis-tical samples of active and evolved galaxies, selected froma wide range of sources can be compiled and diagnosed indetail at these interesting redshifts. Samples can be drawnfrom the very large WISE- and Herschel-selected infraredsamples of galaxies, from EUCLID 24-mag large-area near-infrared galaxy survey, augmented by near-infrared selec-tion in surveys from UKIDSS (whose deepest field reachesapproximately 1 mag deeper than EUCLID wide-area sur-vey in the H band) and VISTA, and in the optical from LSSTand SDSS. Spectra for rare and unusual galaxies and AGNsselected from wide-field imaging surveys can be obtainedusing the wide-field of THESEUS grism, thus building anextensive reference sample for studying the environmentsof the selected galaxies and AGNs, identifying large-scalestructures and allowing overdensities to be measured. Theparallel acquisition of spectral and imaging data over sub-stantial areas would build up a clear picture of the environ-ments, including serendipitiously-selected spectra, all tak-ing advantage of the spectral resolution delivered for THE-SEUS’s primary science of investigating GRB afterglows.

3 Scientific Requirements

THESEUS is designed to achieve two primary scientific goals:

1. Explore the physical conditions of the early Universe byproviding a complete census of GRBs in the first billionyears.

2. Perform an unprecedentedly deep monitoring of the X-ray transient Universe thus playing a fundamental rolein the coming era of multi-messenger and time-domainastrophysics

These goals are very demanding in terms of technology andrequire a combination of on-board capability to perform wide-field X-ray imaging, the ability to obtain broad bandpass X-ray spectra and to localise and characterise the high-energytransients in the optical-IR. The conversion from THESEUSscience goals to instrument and spacecraft requirements areshown in Fig. 22.

To meet the science requirements requires the provisionof three instruments on board: a wide-field soft X-ray mon-itor with imaging capability (the SXI); a harder X-ray, non-imaging spectroscopic instrument with the same field of viewas the SXI (the XGIS); and an optical/near-IR telescope with

26 Amati et al.

Fig. 23 Observer frame peak energy versus bolometric flux of GRBswith well constrained redshifts and spectra detected by various mis-sions with a cloud of points from GRB population simulations(Ghirlanda et al. 2015). Yellow points are those at z>5 The use of asofter X-ray band permits the detection of GRBs with lower fluenceand hence enhances the detection of higher redshift objects.

both imaging and spectroscopic capability (the IRT). Thespacecraft needs to be agile (fast response to enable the IRTto detect the source) and be able to rapidly communicatetriggers to the ground so as to enable other observatories toalso follow-up the new transients. The ability to point thespacecraft into the night sky (anti-solar) direction for partof the orbit enhances rapid ground follow-up capability toprovide additional (multi-wavelength) data. Capability forstable 3-axis pointing for >1 ksec is required to detect thelongest duration transients.

3.1 Wide field monitoring in soft X-rays with deepsensitivity

To greatly increase the rate of GRB detection at high red-shifts and detect large numbers of other transients while si-multaneously providing accurate localisations requires theprovision of a large field-of-view soft X-ray imaging in-strument (see Sect. 4.1). We have performed simulations(Ghirlanda et al. 2015) which show the scientific goals canbe met with an instrument field of view of 1 sr, a sensitiv-ity in 1000 s of 10−10 erg cm−2 s−1 (0.3-6 keV) and imag-ing capability sufficient to provide 0.5-1 arcmin localisations(2 arcmin worst case at 90% c.l.; this requires a PSF FWHM4.5 arcmin).

Multiple timescale software triggers are required to findthe range of flux versus duration transient events. Using suchan instrument (the THESEUS SXI) and taking into accountthe soft X-ray background, Fig. 23 shows the expected an-nual rate of GRBs as a function of redshift. Also plotted isthe rate of GRBs found by Swift (where the redshift dis-tribution has been linearly scaled up based on those withredshift determinations - only approximately one third of

Fig. 24 The annual rate of GRBs predicted for THESEUS SXI (red)compared to Swift (blue). The upper scale shows the age of the Uni-verse. For Swift the actual number of known redshifts is approximatelyone third that plotted and none were determined on board (the bluecurve has been linearly scaled upwards to match the total Swift trig-ger rate). For THESEUS the red region uses the simulations fromGhirlanda et al. (2015) and adopts the expected sensitivity for the SXIand XGIS instruments.

Swift discovered GRBs have redshifts, all determined fromthe ground). The predicted annual rate of GRB detections byTHESEUS SXI is 300-700 per year, with a very high (>5-10) increased rate relative to Swift at the highest redshifts.

As discussed below in the section on IR follow-up, imag-ing and photometric redshifts will be obtained on-board forthe highest redshift GRBs and spectroscopic redshifts forthe majority. For those GRBs detected on board but with-out spectroscopy triggers sent to ground, ground-based tele-scopes can be used to obtain spectra - giving priority tothose with photometric indication of high redshift. THE-SEUS alone will obtain more spectroscopic redshifts on boardin a year than Swift has provided in a decade. The search forhigh-z GRBs is part of a more general unprecedentedly deepmonitoring of the X-ray transient Universe, whose motiva-tion have been detailed in previous sections. The predictedrate of detection of electromagnetic counterparts of GW sig-nals and of other transient and variable source types duringthe survey is shown in Table 1 and 2. The very large detec-tion rate of other transient types is due to the high sensitivityof THESEUS. This is illustrated in Fig. 4, where the sourcedetection sensitivity of the proposed SXI and XGIS instru-ments are plotted verses integration time and overlaid arevarious sources types.

3.2 Provision of broad-band X-ray spectroscopy

The scientific objectives of THESEUS require the secureidentification of sources types, in particular GRB triggers.The soft X-ray instrument is the primary source locator andhas a high sensitivity to a wide variety of source types, as

THESEUS 27

discussed above, as required to achieve the scientific goals.This instrument will trigger on a large number of knownsources which should not result in a spacecraft slew, butalso other transient sources only some of which are GRBs.To reliably identify GRBs as well as spectroscopically char-acterise other sources and reduce the number of demandedslews requires the provision of a sensitive broad-band X-gamma ray instrument well matched to the sensitivity of thesoft X-ray instrument and with source location capabilitiesof a few arcmin in the X-ray energy band. To meet the scien-tific goals we require an instrument with the following char-acteristics:

– extend the soft X-ray band of the imaging instrument upto the MeV band;

– identify and localize with a few arcmin accuracy theGRBs by providing simultaneous triggers and by provid-ing higher energy light curves and spectra to determinethe luminosity of the GRB;

– reduce the demanded number of spacecraft slews to ob-serve with the IRT and act as a crucial filter to reduce softX-ray trigger volume for ground/space telescope follow-up;

– measure unbiased GRB/transient X- and gamma-ray spec-tra down to short time scales (ms time scales for thestrongest events) to probe GRB physics.

The proposed XGIS instrument provides the required sen-sitivity and bandpass (Fig. 25). The sensitivity of the SXIand XGIS are well matched over the typical durations ofGRBs (few to few tens of seconds). The XGIS will providespectroscopy over 2-20000 keV with monitoring timescalesdown to milli-seconds. Despite advances during the Swiftand Fermi era to identify and characterise GRB phenomenonrequires study of the prompt emission. Planned future mis-sions (e.g. SVOM) do not provide the required combinationof sensitivity and bandpass.

3.3 Optical-IR follow-up

The scientific goals of THESEUS require the following on-board capabilities for an optical/near IR telescope (IRT) tofollow-up GRBs after a demanded spacecraft slew:

1. Identify and localize the GRBs found by the SXI andXGIS to arcsecond accuracy in the visible and near IRdomain (0.7-1.8 µm);

2. Autonomously determine the photometric redshift of GRBsfor z>4 and provide redshift upper limits for those atlower redshift;

3. Provide precise spectroscopic redshift measure for brightGRBs, together with limits on the intrinsic NH and metal-licity for the majority of GRBs at z>4.

Table 3 THESEUS yearly detection and redshift measurements rates.The redshift measurements indicated are those that would be achieveddirectly on-board and do not include refined and/or additional mea-surements on-ground (as it is currently done for other observatories asSwift). Note that photometric redshifts are possible only at z&5, whenthe Lyman “dropout” or “break” gets inside the IRT band.

THESEUS GRB/yr All z>5 z>8 z>10

Detections 387-870 25-60 4-10 2-4Photometric z 25-60 4-10 2-4Spectroscopic z 156-350 10-20 1-3 0.5-1

The requirement number 1 is justified by the fact that thegoal of the THESEUS mission is to study the Universe atz > 6 in order to study the epoch of reionization. CMB ex-periments suggest that reionization was underway at z∼9,while it appears to be completed by z∼6.5. The questionis whether massive stars could sustain a largely reionizedUniverse at z=6-9, and beyond. GRB afterglow spectra arepower laws and, due to the Ly-alpha drop-out (i.e. the Lymanalpha absorption within the GRB host galaxies and inter-vening IGM), a very attenuated signal is expected at wave-lengths shorter than the Lyman alpha break, providing anunmistakable feature. Due to cosmological expansion theLy-alpha wavelength (1216 Å=0.126 µm) moves along theenergy band, and in order to measure GRB redshifts betweenz=4 and z=10 the telescope detector has to be sensitive in the0.7 to 1.8 µm range. GRBs with z<4 can be used as compar-ison to evaluate how massive stars evolve along the historyof the Universe, and they can be easily followed up fromground. It is for high redshift GRBs that a NIR telescope inspace really takes advantage of the absence of backgrounddue to the atmosphere. The field of view of the IRT tele-scope shall be larger than 4x4 arcmin, given that the SXI willprovide error boxes which are at worst 2 arcmin (90% c.l.)radius. For requirement number 2 the telescope will be op-erated in low resolution mode (R∼10-20), and the Ly-alphadrop-off will be searched for. A fit of the sources’ low res-olution spectra, done on-board, should be capable of identi-fying high-redshift candidates. If we focus on GRBs at red-shift larger than 6 the detector shall be optimized in termsof QE the 0.8-1.5 µm wavelength range. To obtain reliableresults the detector QE shall be known within 10-20%. Therequirement number 3 deals with the high-resolution spectra(R∼500). As shown in the left panel of Fig. 26, a resolutionof the order of R∼500 is good enough to identify the mainabsorption lines in bright GRB NIR afterglow spectra. Suchdetections would (i) enable a more precise measure of theGRB redshift, that goes beyond the “mere” detection of theLy-alpha drop off achievable with lower resolution spectraand (ii) help in discriminating between highly-extinguishedand high-redshift (z>5) events. Besides the redshift mea-sure, with the IR resolution spectroscopy it will be possibleto derive limits on element relative abundances and metallic-

28 Amati et al.

Fig. 25 The left-hand panel shows the GRASP (FOV×Effective Area) of the THESEUS/SXI in the soft X-ray energy band compared to XMM-Newton and eROSITA. The GRASP of X-ray monitors on-board MAXI and ASTROSAT are also show for completeness, even though these arenot focusing and their sensitivity for a given effective area is substantially sworse than that of focusing teelscopes. The leap in monitoring/surveyof the soft X-ray sky allowed by THESEUS/SXI is outstanding. The right panel shows THESEUS/XGIS filling the parameter space in the top-leftcorner of the right-hand panel where other instruments have either too high an X-ray threshold or too low effective area, and will still provide1000-1550 cm2 effective area up to several MeVs (Yonetoku et al. 2014).

Fig. 26 Left: a simulated IRT high resolution (R=500) spectrum for a GRB at z=6.3 observed at 1 hr post trigger assuming a GRB similar toGRB 050904. The spectrum has host log(NH)=21 and neutral fraction Fx=0.5 (and metallicity 0.1 solar). The two models are: Red: log(NH)=21.3,Fx=0 Green: log(NH)=20.3, Fx=1. The IRT spectra provide accurate redshifts. Right: simulated IRT low resolution (R=20) spectra as a functionof redshift for a GRB at the limiting MAB=20.8 mag at z=10 (brighter bursts can be expected at this redshift), and by assuming a 20 min exposure.The underlying (noise-free) model spectra in each case are shown as smooth, dashed lines. Even for difficult cases the low-res spectroscopy shouldprovide redshifts to a few percent precision or better. For many applications this is fine - e.g. star formation rate evolution.

ity (together with a measure of NH for the brightest events,obtained by fitting the red wing of the Ly-alpha). Such in-formation will be vital to optimize ground-based follow-up(with the large aperture facilities that will be available in2028, like ELT), aimed at precise optical/IR spectroscopicstudies for a detailed characterization of the GRB environ-ment throught the measure of chemical abundances, metal-licity, and SFR.

The sensitivity of the SXI triggering system in the 0.3-6 keV energy band probes a fluence range of 10−8 and 10−9

erg cm−2. Based on (Ghirlanda et al. 2015) the proposedTHESEUS IRT with a limiting sensitivity of 20.6 mag inthe H (1.6±0.15 µm) filter is expected to detect all the GRBcounterparts in imaging and low-resolution spectroscopy, ifpointed early after the GRB trigger, in a 300 s exposure,and for the large majority of GRBs high-resolution spectra

THESEUS 29

can be taken even 1-2 hours after the GRB (first or secondspacecraft orbit) with the 19th magnitude sensitivity IRT.

In Fig. 2 we show the rate of GRBs whose redshift willbe spectroscopically determined by THESEUS on-board asa function of redshift. For the other GRBs detected by theSXI/XGIS, the IRT will provide a location and a redshiftlimit and thus provide a redshift estimate for the entire sam-ple detected on-board. The cumulative distribution repre-sents the rate (number of GRBs per year) that can be de-tected by THESEUS (red solid filled region). The width ofthe distribution accounts for the uncertainties of the popu-lation synthesis code adopted. For comparison, the rate ofdetection of GRBs by Swift is shown with a blue line. THE-SEUS out-performs Swift by about an order of magnitude atall redshifts and by more at the highest redshifts. Using theIRT to follow-up the SXI and XGIS will identify the high-est priority high-redshift targets for the early Universe sci-ence goals. This rate is derived from the actual population ofGRBs detected by Swift and with measured redshift multi-plied by a factor of 3. Indeed, approximately only 1/3 of theGRBs detected by Swift have their redshift measured. Theupper axis shows the age of the Universe (5 Lobster mod-ules with focal length 300 mm and individual field of view0.16 sr are assumed). The detection and redshift-estimateannual rates expected form THESEUS are also summarizedin Table 3.

4 Scientific instruments

Following the scientific requirements described in the pre-vious section, the baseline Instrument suite configuration ofTHESEUS payload includes 4 lobster-eye modules (F=300 mm),a 70 cm class IR telescope and 3 X-ray/soft gamma-ray coded-mask cameras based on Si+CsI(Tl) coupling technology cov-ering twice the FOV of the lobster-eye modules. In sum-mary, the scientific payload of THESEUS will be composedof:

– Soft X-ray Imager (SXI): a set of 4 “Lobster-Eye” X-ray (0.3-6 keV) telescopes covering a total FOV of ∼1 srwith 0.5-1 arcmin source location accuracy, provided bya UK led consortium;

– InfraRed Telescope (IRT): a 70 cm class near-infrared(up to 2 µm) telescope with imaging and moderate spec-tral capabilities provided by a France led consortium (in-cluding ESA, Switzerland, and Germany);

– X-Gamma ray Imaging Spectrometer (XGIS): a spec-trometer comprising 3 detection units based on SDD+CsI(Tl)modules (2 keV-20 MeV), covering twice the FOV of theSXI. This instrument will be provided by an Italian ledconsortium (including Spain).

All instruments are equipped with an Instrument DataHandling Unit (I-DHU), interfacing each of the three instru-

Table 4 SXI detector unit main physical characteristics.

Energy band (keV) 0.3-6Telescope type Lobster eyeOptics aperture 320×320 mm2

Optics configuration 8×8 square pore MCPsMCP size 40×40 mm2

Focal length 300 mmFocal plane shape sphericalFocal plane detectors CCD arraySize of each CCD 81.2×67.7 mm2

Pixel size 18 µmNumber of pixel 4510×3758 per CCDNumber of CCDs 4Field of View ∼1 srAngular accuracy (best, worst) (<10, 105) arcsec

ments with the spacecraft (provided by a German led con-sortium and Poland). The general avionic block diagram ofthe THESEUS PLM as well as SVM is shown in Fig. 42.

4.1 The Soft X-Ray Imager (SXI)

The THESEUS Soft X-ray Imager (SXI) comprises 4 detec-tor units (DUs). Each DU is a wide field lobster eye tele-scope using the optical principle first described by Angel(1979) with an optical bench as shown in Fig. 27.

The optics aperture is formed by an array of 8×8 squarepore Micro Channel Plates (MCPs). The MCPs are 40×40 mm2

and are mounted on a spherical frame with radius of curva-ture 600 mm (2 times the focal length of 300 mm). Table 4summarizes the SXI characteristics. The mechanical enve-lope of a SXI module has a square cross-section 320×320 mm2

at the optics end tapering to 200×200 mm2 at detector. Thedepth of the detector housing is 200 mm giving an over-all module length of 500 mm. The left-hand side of Fig. 28shows the optics frame of the breadboard model for the SVOMMXT lobster eye telescope which comprises 21 square MCPsmounted over a 5×5 grid (the corners are unoccupied forthis instrument). The front surface is spherical with radius ofcurvature 2000 mm giving a focal length of 1000 mm. Thedesign proposed for SXI uses the same plate size and ex-actly the same mounting principle but a shorter focal length,300 mm, so the radius of curvature of the front surface mustbe 600 mm. The right-hand panels of Fig. 28 shows a schematicof a single plate and a micrograph that reveals the squarepore glass structure. The focal plane of each SXI module isa spherical surface of radius of curvature 600 mm situated adistance 300 mm (the focal length) from the optics aperture.The detectors for each module comprise a 2×2 array of largeformat detectors tilted to approximate to the spherical focalsurface.

CalibrationThe following calibrations are envisaged:

30 Amati et al.

Fig. 27 The SXI block diagram concept (left) and optical elements (right).

– SXI optic: X-ray beam line testing to measure the focallength, the effective area and the point spread functionas a function of off-axis angle and energy.

– SXI detector: Vacuum test facility to measure the gainand energy resolution as a function of energy.

– SXI end-to-end: X-ray beam line facility confirm the fo-cal length and measure the instrument effective area andPSF as a function of photon energy and off-axis angle.

– SXI in orbit calibration: use cosmic sources to confirmin-flight alignment, plate scale, point spread function, ef-fective area, vignetting and energy resolution. A regularmonitoring of cosmic sources is planned to verify thestatus and correctness of the instrument calibration.

SXI Performance, Sensitivity and Data RateThe imaging area of the CCDs sets the field of view of eachmodule. A compliment of 4 SXI modules has a total fieldof view of 3200 square degrees (0.9 sr). The point spreadfunction is shown in Fig. 29. The inner dotted square showsthe off-axis angle at which the cross arms go to zero as de-termined by the L/d ratio of the pores. For optimum perfor-mance at 1 keV we require L/d=50. The outer dotted squareindicates the shadowing of the cross-arms introduced by thegap between the individual MCPs in the aperture. The cen-tral true-focus spot is illustrated by the projection plot to theleft. The FWHM is 4.5 arcmin and all the true-focus fluxis contained by a circular beam of diameter 10 arcmin. Thecollecting area, within a 10 arcmin beam centered on thecentral focus, as a function of energy is shown in Fig. 30.The optics provides the area plotted in black. The red curveincludes the quantum efficiency of the CCD and the trans-mission of the optical blocking filter comprising a 60 nmAluminium film deposited over the front of the MCPs and260 nm of Aluminium plus 500 nm of parylene on the sur-face of the detectors. Because the angular width of the opticsMCP-array is 2.3 deg larger than the CCD-array the field ofview is unvignetted at 1 keV and above so the collectingarea shown in Fig. 30 is constant across the field of view.Using the Rosat All-sky Survey data we can estimate the

count rate expected from the diffuse sky (Galactic and Cos-mic) and point sources.

The sensitivity to transient sources using this backgroundrate and a false detection probability of 1.0×10−10 is shownas a function of integration time in Fig. 4. For longer in-tegration times the source count required rises, e.g., to 30counts for a 1 ks integration. We find that 94% of the SwiftBAT bursts (before 2010 September 16) would be detectedby the SXI. The X-ray light curve of the afterglow would bedetected to >1 ks after the trigger for a large fraction of thebursts.

Trigger AlgorithmThe ideal algorithm would be some form of matched filter-ing using the full PSF distribution but because of the extentof the PSF this would be far too computationally heavy. Atthe other extreme a simple scheme would be to search forsignificant peaks using the cell size commensurate with thecentral peak in the PSF. This would be faster but utilizesonly ∼25% of the total flux detected. The scheme describedbelow is a two stage process which exploits the cross-armgeometry but avoids computationally expensive 2-D cross-correlation. For the 1st stage the focal plane is divided intosquare patches with angular side length ∼ 4d/L = 1/12 ra-dians aligned with the cross-arm axes. The dotted centralsquare shown in Fig. 31 indicates the size and orientation ofsuch a patch. The optimum size of such patches depends onthe HEW of the lobster-eye optic and the background countrate. The patches could correspond to detector elements ortiles in the focal plane, e.g. CCD arrays. The peak profile isthe line spread profile of the central spot and cross-arms ofthe PSF. The remainder of the histogram distribution arisesfrom events in the cross-arm parallel to the histogram di-rection, the diffuse component of the PSF and any diffusebackground events not associated with the source. Becausewe are looking for transient sources the fixed pattern of thesteady sources in the field of view at the time would have tobe subtracted from the histogram distributions. As the point-ing changes this fixed pattern background would have to be

THESEUS 31

Fig. 28 Left: The SVOM MXT lobster eye optic aperture frame. Top right: A schematic of a single square pore MCP. Bottom right: A micrographof a square pore MCP showing the pore structure. This plate has a pore size d=20 µm and a wall thickness w=6 µm.

Fig. 29 The point spread function of the SXI.

updated. A transient source is detected if a significant peakis seen in both histograms.

The sensitivity of detection and accuracy of the derivedposition of the source within the patch depends on the binsize of the histograms, the HEW of the central peak of thePSF and the background. For the most sensitive detectionthe bin size should be approximately equal to the HEW butthis will limit the accuracy of the position. If the bin size ofthe histograms is chosen to be significantly smaller than thewidth of the HEW then the histograms can be smoothed bycross-correlation with the expected line width profile of thepeak-cross-arm combination. Using the smaller bin size thehistograms can also be used to estimate the position centroidof the source within the patch. The significance used for thisfirst stage should be low, e.g. 2.5 σ. This will provide candi-date positions for the second stage.

For each of the candidate positions identified in the firststage a cross-arm patch is set up to cover just the detec-tor area which is expected to contain a fraction of the fullcross-arms and the central peak in the PSF. The cross-patchdimensions are changed depending on the integration time∆T. For short integration times the total background countwill be small and the cross-patch size is set large to capturea large fraction of the counts from the cross-arms and centralpeak.

The above considers a single value of ∆T. We envisagethat a series of searches would be run in parallel each usinga different integration time so that the sensitivity limit as afunction of ∆T is covered. The basic scheme is illustrated inFig. 31. The total source count assumed for this illustrationwas 40 spread over the full PSF as plotted in Fig. 29. The binsize used for the histograms was 1 arcmin, and the HEW ofthe central peak of the PSF is approximately 4 arcmins. Wehave tested the algorithm over a range of integration timesand background conditions. It achieves the sensitivity limitplotted in the science requirements section. When a signif-icant transient peak is identified the position must be con-verted to sky coordinates using the current aspect solution(from Payload star trackers). Positions of all transients foundmust be cross-correlated with known source catalogues, e.g.Rosat All-Sky Survey, Flare stars, Swift BAT catalogue etc.Any position which does not match a known position mustbe passed to the Space Craft as a potential trigger position.

The processing required to implement the above is asfollows:

1. Extract frames from the CCD at ∆T=2 s (this is the fastestrate set by the frame time).

2. Apply event reconstruction algorithm to the frames togive an event list with positions in CCD pixels and apulse height.

3. Convert the pixel positions into a local module coordi-nate frame which is aligned to the cross-arms of the PSF.

4. Accumulate counts in the 1-D histograms.5. Subtract the fixed source/background pattern from the

histograms.6. Scan the histograms for significant peaks and extract can-

didate positions for further analysis.7. Set up the cross-arm mask at candidate positions to look

for significant peaks. Calculate an accurate position inthe local module coordinate frame for the peak.

8. Convert this position into global sky coordinates (quater-nion).

32 Amati et al.

Fig. 30 Left: Collecting area as a function of energy (assuming a focal lenght of 300 mm and including the contributions of the central spot, the2 cross-arms, and the straight through flux). The black line represents the optics only. The red curve includes the quantum efficiency of the CCDand the transmission of the optical blocking filter. Right: The position accuracy of the SXI as a function of source and background count. R90 isthe error radius that contains 90% of the derived positions.

9. Check positions against on-board catalogues to weed outknown sources.

10. Communicate unidentified transients to the Spacecraft.

Note that points 4-7 above must be repeated for different ∆Tvalues (e.g., 2, 20, 200, 2000 s).

4.2 The X-Gamma ray Imaging Spectrometer (XGIS)

The X-Gamma ray Imaging Spectrometer (XGIS) comprises3 units (telescopes). The three units are pointed at offset di-rections in such a way that their FOV partially overlap. Eachunit (Fig. 32 and Table 5) has imaging capabilities in thelow energy band (2-30 keV) thanks to the combination of anopaque mask superimposed to a position sensitive detector.A passive shield placed on the mechanical structure betweenthe mask and the detector plane will determine the FOVof the XGIS unit for X-rays up to about 150 keV energy.Furthermore the detector plane energy range is extended upto 20 MeV without imaging capabilities. The main perfor-mance of an XGIS unit are reported in Table 6. The detec-tion plane of each unit is made of 4 detector modules eachone about 195×195×50 mm in size detecting X and gammarays in the range 2 keV-20 MeV. For each energy loss in themodule, whatever procured by EM radiation or ionizing par-ticle, the energy released, the 3 spatial coordinates and theenergy deposit of the interaction and time of occurrence willbe recorded.

The basic element of a module (Fig. 32) is a bar madeof scintillating crystal 5×5×30 mm3 in size. Each extremeof the bar is covered with a Photo Diode (PD) for the read-

out of the scintillation light, while the other sides of the barare wrapped with a light reflecting material conveying thescintillation light towards the PDs. The scintillator materialis CsI(Tl) peaking its light emission at about 560 nm. ThePD is realized with the technique of Silicon Drift Detec-tors (SDD-PD; Gatti and Rehak 1984) with an active areaof 5×5 mm2 matching the scintillator cross section. Crys-tals are tightly packed in an array of 32×32 elements toform the module. The SDD and scintillator detect X- andgamma-rays. The operating principle (see Fig. 32 right) isthe following. The top SDD-PD, facing the X-/gamma-rayentrance window, is operated both as X-ray detector for lowenergy X-ray photons interacting in Silicon and as a read-outsystem of the scintillation light resulting from X-/gamma-ray interactions in the scintillator. The bottom SDD-PD atthe other extreme of the crystal bar operates only as a read-out system for the scintillations. The discrimination betweenenergy losses in Si and CsI is based on the different shapeof charge pulses. While the electron-hole pair creation fromX-ray interaction in Silicon generates a fast signal (about10 ns rise time), the scintillation light collection is domi-nated by the fluorescent states de-excitation time (0.68 µs,64%, and 3.34 µs, 36%, for CsI(Tl)) and a few µs shap-ing time is needed in this case to avoid significant ballisticdeficit. Pulse shape analysis (PSA) techniques are adoptedto discriminate between signals due to energy losses in Si orCsI. The results we obtained for the separation of the energylosses in the case of an 241Am source (Marisaldi et al. 2004)are shown in Fig. 34. As can be seen from the left panelof this figure, the ratio of the two pulse heights is approxi-mately constant for pulses of common shape and allows dis-

THESEUS 33

Fig. 31 The two stage trigger algorithm. Top left-hand panel: the detected event distribution ∆T=4 seconds and a source count rate of 40 cts s-1over the full PSF. The cross-arms are rotated wrt the detector axes to demonstrate how this can be handled. Top right-hand panel: the detectedevent distribution in the patch of sky aligned to the cross-arm axes of the PSF (shown as the red rectangle in the top left-hand panel). The redcross-patch indicates the area used for the second stage of the algorithm. Bottom panels: the histograms along columns and rows in the patch.

Table 5 XGIS detector unit main physical characteristics.

Energy band 2 keV-20 MeVDetection plane modules 4detector pixel/module 32×32pixel size (= mask element size) 5×5 mmLow-energy detector (2-30 keV) Silicon Drift Detector 450 µm thickHigh energy detector (>30 keV) CsI(Tl) 3 cm thickDiscrimination Si/CsI(Tl) detection Pulse shape analysisDimension 50×50×85 cmPower 30.0 WMass 37.3 kg

Table 6 XGIS unit characteristics vs energy range.

2-30 keV 30-150 keV >150 keV

Fully coded FOV 9×9 deg2

Half sens. FOV 50×50 deg2 50×50 deg2 (FWHM)Total FOV 64×64 deg2 85×85 deg2 (FWZR) 2πsrAng. res 25 arcminSource location accuracy ∼5 arcmin (for >6 σ source)Energy res 200 eV FWHM at 6 keV 18% FWHM at 60 keV 6% FWHM at 500 keVTiming res. 1 µs 1 µs 1 µsOn axis useful area 512 cm2 1024 cm2 1024 cm2

34 Amati et al.

Fig. 32 Left: Sketch of the XGIS Unit. Right: Principle of operation of the XGIS detection units: low-energy X-rays interact in Silicon, higherenergy photons interact in the scintillator, providing an energy range covering three orders of magnitude. A pulse shape discriminator determinesif the interaction has occurred in Si or in the crystal.

Fig. 33 Sketch of the XGIS module. A module (right) is made of anarray of 32×32 scintillator bars with Si PD read-out at both ends (left).Both PD and scintillators are used as active detectors. The PDs readoutelectronics consist of an ASIC pre-amp mounted near each PD’s anodewhile the rest of the processing chain is placed at the module sides andbottom.

crimination between interactions taking place in Silicon orin the scintillator. For gamma-rays interacting in the scintil-lator, combining the signals from the two PDs at the extremeof each bar allows to determine the energy and the depth ofthe interaction inside the crystal (Labanti et al. 2008).

XGIS building blocksIn the XGIS HW, the main building blocks (see Fig. 35 forone XGIS unit) are:1. the mask and the FOV delimeter;2. the scintillator detectors;3. the FEE in both its analogue part (with SDD-PD, ASIC1

and ASIC2), digital part (DFEE) and services (TLM,TLC Power supply).The coded mask of each XGIS unit, placed 70 cm above

the detector modules is made of stainless steel of 0.5 mmthick. The mask overall size is 50×50 cm and will have apattern allowing self-support in order to guarantee the max-imum transparency of the open elements. The mechanicalstructure connecting the mask with the detector is also madeof stainless steel 0.1 mm thick and supports 4 Tungsten slats45 cm high with a variable thickness (0.5-0.3 mm). Thisstructure will act as a lateral passive shield for the imagersystem (1-30 keV) and as a FOV delimiter at energies>150 keV.In the latter range, the resulting FOV is 50×50 deg2 (FWHM)and 85×85 deg2 (FWZR). By combining the three units,with an offset of ±35 deg for two of them, the FOV delimiterguarantees an average XGIS effective area of ∼1400 cm2 inthe SXI FOV (104×31 deg2).

Data Handling Unit (DHU) and its functionsThe whole XGIS background data rate (3 units) towards theDHU is of the order of 6000 event/sec in the 2-30 keV rangeand about 3700 event/sec above 30 keV. Each event receivedby DHU will be identified with a word of 64 bits (4 for mod-ule address, 10 for bar address, 10 for signal amplitude ofthe fast top channel, 11 for signal amplitude of the slow topchannel, 11 for signal amplitude of the slow bottom channel,18 for time). DHU functions will be:

THESEUS 35

Fig. 34 Left: bidimensional spectrum of a 241Am source showing interaction in Silicon (top line) and in the scintillator (bottom line). Right:distribution of the ratio of the two processing chains for events in Silicon and in the scintillator. The large peak separation indicates the optimaldiscrimination performance.

– discriminates between Si and CsI events.– For CsI events, evaluates the interaction position inside

the bar by weighting the signals of the 2 PDs (a few mmresolution expected). Combining this information withthe address of the bar (5×5 mm in size) each modulebecomes a 3D position sensitive detector.

– Exploiting the 3D capabilities background can be mini-mized.

– It continuously calculates along the orbit the event rateof each module in different energy bands (typically 2-30 keV, 30-200 keV and >200 keV) and on 5 differenttimes scales (e.g., 1 ms, 10 ms, 100 ms, 1 s, and 10 s).

– In the 2-30 keV range and for each unit, it produces im-ages of the FOV in a defined integration time.

– It holds in a memory buffer all the XGIS data, rates andimages of the last 100 s (typical) with respect to the cur-rent time.

– Produce maps of the three unit planes with event pixelby pixel histograms in different energy bands (typically32 with E width varying logarithmically) and with se-lectable integration times (min 1 s).

XGIS and the GRB trigger systemXGIS will contribute to the THESEUS’s GRB trigger sys-tem in different ways:

1. Qualification of the SXI triggers. The primary role ofXGIS is to qualify the SXI triggers as true GRB. Thebasic algorithm for GRB validation is based on an eval-

uation of the significance of the count rate variation, cal-culated as described in the sub-section below. The pro-cedure will be the following:

– from the SXI direction given to the event, it is iden-tified one of the three XGIS units in which the eventhas potentially been detected;

– look for an excess of the rates in the modules of thisunit in the bands 2-30 keV and 30-200 keV with re-spect to the average count-rate continuously calcu-lated by DHU.

2. Autonomous XGIS GRB trigger based on data rate. Theautonomous GRB trigger for XGIS inherits the experi-ence acquired with the Gamma Ray Burst Monitor (GRBM)aboard BeppoSAX and that acquired with the Mini-calorimeteraboard AGILE (Fuschino et al. 2008) and concerns allthe modules of the 3 XGIS units. For each module theabove energy intervals (2-30 and 30-200 keV) are con-sidered for the trigger. The mean count rate of each mod-ule in each of these bands is continuously evaluated ondifferent time scales (e.g., 10 ms, 100 ms, 1 s, and 10 s).A trigger condition is satisfied when, in one or both ofthese energy bands, at least a given fraction (typically&3) of detection modules sees a simultaneous excesswith a significance level of typically 5 σ on at least onetime scale with respect to the mean count rate.

3. Autonomous XGIS GRB trigger based on images. Foreach XGIS unit, the 2-30 keV actual images will be con-fronted with reference images derived averaging n (typi-

36 Amati et al.

cally 30) previous images, and a spot emerging from thecomparison at a significance level of 5 σ typically willappear. If one of the above trigger condition is satisfied,event by event data, starting from 100 s before the trig-ger are transmitted to ground, the duration of this modelasting until the counting rate becomes consistent withbackground level.

Telemetry requirementsFor the study of transient or persistent sources different trans-mission mode will be selected starting from the photon listand the histogram maps of the units. Typically the TLM loadwill be maintained below 2 Gbit/orbit transmitting: (i) at lowenergies (<30 keV) pixel by pixel histograms in various En-ergy channels (e.g. 32 ch) with variable integration times(e.g. 64 sec); (ii) above the 30 keV the whole photon list.In particular observations (e.g., crowded fields) a photon byphoton transmission in the whole energy range will be se-lected for a total maximum telemetry load of 3 Gbit/orbit.In the case of a GRB trigger all the information availablephoton by photon is transmitted with a maximum telemetryload of 1 Gbit.

XGIS sensitivityThe 5 σ XGIS sensitivity for an integration of 1 s with en-ergy in the SXI FOV is shown in Fig. 36, along with theXGIS flux sensitivity versus observation time at a signifi-cance of 5 σ in different energy ranges. In Fig. 37, the FOVof the XGIS in the 2-30 keV band is compared with the SXIFOV, and the XGIS sensitivity vs. GRB peak energy is com-pared with that of other instruments.

4.3 The InfraRed Telescope (IRT)

The InfraRed Telescope (IRT) on board THESEUS is de-signed in order to identify, localize and study the transientsand especially the afterglows of the GRBs detected by theSoft X-ray Imager (SXI) and the X and Gamma ImagingSpectrometer (XGIS). The telescope (optics and tube assem-bly) can be made of SiC, a material that has been used inother space missions (such as Gaia, Herschel, Sentinel2 andSPICA study). Simulations using a 0.7 m aperture Cassegrainspace borne NIR telescope (with a 0.23 m secondary mirrorand a 10×10 arcmin imaging flied of view), using a spacequalified Teledyne Hawaii-2RG (2048×2048 pixels) HgCdTedetector (18 µm/pixels, resulting in 0.3 arcsec/pix plate scale)show that for a 20.6 (H, AB) point like source and 300 s in-tegration time one could expect a SNR of ∼5. The telescopesensitivity is limited by the platform jitter. In addition, due tothe APE capability of the platform (2 arcmin), the high reso-lution spectroscopy mode cannot make use of a fine slit, anda slit-less mode over a 5×5 arcmin area of the detector will

be implemented (similarly to what is done for the WFC3 onboard the Hubble Space Telescope), with the idea of mak-ing use of the rest of the image to locate bright sources inorder to correct the frames a posteriori for the telescope jit-ter. The same goal could also be obtained by making useof the information provided by payload the high precisionstar trackers mounted on the IRT. Hence the maximum lim-iting resolution that can be achieved by such a system forspectroscopy is limited to R∼500 for a sensitivity limit ofabout 17.5 (H, AB) considering a total integration time of1800 s. The IRT expected performances are summarized inTable 7. In order to achieve such performances (i.e., in con-ditions such that thermal background represents less than20% of sky background) the telescope needs to be cooled at240±3 K, and this can be achieved by passive means. Con-cerning the IRT camera, the optics box needs to be cooledto 190±5 K and the IR detector itself to 95±10 K: this al-lows the detector dark current to be kept at an acceptablelevel. The cooling of the detector at these low temperaturescan hardly be achieved with a passive system in a low Earthorbit such as the one foreseen for THESEUS, due to the ir-radiation of the radiators of the infrared flux by the Earth at-mosphere. A TRL 5 cooling solution for space applicationsis represented by the use of a Miniature Pulse Tube Cooler(MPTC).

In order to keep the camera design as simple as possi-ble (e.g., avoiding to implement too many mechanisms, liketip-tilting mirrors, moving slits, etc.), we could implementa design with an intermediate focal plane making the inter-face between the telescope provided by ESA/industry andthe IRT instrument provided by the consortium, as shown inthe block diagram in Fig. 38. The focal plane instrument iscomposed by a spectral wheel and a filter wheel in whichthe ZYJH filters, a prism, and a volume phase holographic(VPH) grating will be mounted, in order to provide the ex-pected scientific product (imaging, low and high-resolutionspectra of GRB afterglows and other transients).

Specifications of the entire system are given in Table 7.The mechanical envelope of IRT is a cylinder with 80 cm di-ameter and 180 cm height. A sun-shield is placed on top ofthe telescope baffle for IRT straylight protection. The ther-mal hardware is composed by a pulse tube cooling the De-tector and FEE electronics and a set of thermal straps ex-tracting the heat from the electronic boxes and camera op-tics coupled to a radiator located on the spacecraft structure.The overall telescope mass is 112.6 kg and the total powersupply is 95 W.

IRT Observing sequenceThe IRT observing sequence is as follow:

1. The IRT will observe the GRB error box in imagingmode as soon as the satellite is stabilized within 1 arcsec.Three initial frames in the ZJH-bands will be taken (10 s

THESEUS 37

Fig. 35 XGIS unit main building blocks.

Fig. 36 Left: XGIS sensitivity vs. energy for an integration of 1 s. Right: XGIS sensitivity as a function of exposure time in different energy bands.

Fig. 37 Left: fractional variation of the effective area in the FOV of the XGIS. Right: Sensitivity of the XGIS to GRBs in terms of minimumdetectable photon peak flux in 1s (5 σ) in the 1-1000 keV energy band as a function of the spectral peak energy (a method proposed by Band2003). As can be seen, the combination of large effective area and unprecedented large energy band provides a much higher sensitivity w/r toprevious (e.g., CGRO/BATSE), present (e.g., Swift/BAT) and next future (e.g., SVOM/ECLAIRS) in the soft energy range, while keeping a verygood sensitivity up to the MeV range.

each, goal 19 AB 5 σ sensitivity limit in H) to estab-lish the astrometry and determine the detected sourcescolours.

2. IRT will enter the spectroscopy mode (Low ResolutionSpectra, LRS) for a total integration time of 5 min (ex-pected 5 σ sensitivity limit in H 18.5 (AB)).

3. Sources with peculiar colours and/or variability (such asGRB afterglows) should have been pinpointed while thelow-res spectra were obtained and IRT will take a deeper(20 mag sensitivity limit (AB)) H-band image for a to-tal of 60 s. These images will be then added/subtractedon board in order to identify bright variable sources with

38 Amati et al.

Table 7 IRT specifications.

Telescope type CassegrainPrimary and Secondary size 700 mm & 230 mmMaterial SiC (for both optics and optical tube assembly)Detector type Teledyne Hawaii-2RG 2048×2048 pixels (18 µm each)Imaging plate scale 0”.3/pixelField of view 10×10 arcmin 10×10 arcmin 5×5 arcminResolution (λ/∆λ) 2-3 (imaging) 20 (low-res) 500 (high-res), goal 1000Sensitivity (AB mag) H = 20.6 (300 s) H = 18.5 (300 s) H = 17.5 (1800 s)Filters ZYJHPrism VPH gratingWavelength range 0.7-1.8 µm (imaging) 0.7-1.8 µm (low-res) 0.7-1.8 µm (high-res, TBC)Total envelope size 800 (diameter) ×1800 mmPower 115 W (50 W for thermal control)Mass 112.6 kg

Fig. 38 The IRT Telescope block diagram concept.

one of them possibly matching one of the peculiar colourones. NIR catalogues will also be used in order to ex-clude known sources from the GRB candidates.– In case a peculiar colour source or/and bright (<17.5

H (AB)) variable source is found in the imaging step,the IRT computes its redshift (a numerical value if5<z<10 or an upper limit z<5) from the low resolu-tion spectra obtained at point (1) and determines itsposition. Both the position and redshift estimate willbe sent to ground for follow-up observations. The de-rived position will then be used in order to ask thesatellite to slew to it so that the source is placed inthe high resolution part of the detector plane (see be-low) where the slit-less high resolution mode spectraare acquired. Following the slew, the IRT enters theHigh Resolution Spectra (HRS) mode where it shallacquire at least three spectra of the source (for a to-tal exposure time of 1800 s) covering the 0.7-1.8 µmrange. Then it goes back to imaging mode (H-band)for at least another 1800 s. Note that while acquiringthe spectra, continuous imaging is performed on therest of the detector (Fig 38). This will allow to the onboard software to correct the astrometry of the indi-vidual frames for satellite drift and jitter and allow a

final correct reconstruction of the spectra by limitingthe blurring effects.

– In case that a faint (>17.5 H (AB)) variable source isfound, IRT computes its redshift from the low reso-lution spectra, determines its position and sends bothinformation to the ground (as for 3a). In this case IRTdoes not ask for a slew to the platform and stays inimaging mode for a 3600 s time interval to establishthe GRB photometric light curve (covering any pos-sible flaring) and leading the light curve to be knownwith an accuracy of <5%.

4.4 The Instrument Data Handling Units (I-DHUs)

Following the concept behind the organization of the THE-SEUS instruments as well as the decentralized avionic scheme,each of the three instrument payloads will be equipped witha dedicated Instrument Data Handling Unit (I-DHU) thatwill serve as their TM/TC and power interface to the space-craft. The aim of this scheme is to provide sufficient com-puting power and data storage to the individual instrumentsand thus to realize a decentralized data handling function.

The mechanical and electrical design of the I-DHUs (de-scribed below) will be the same for all three instruments.Also the operating system and basic software that is runningon the Processor Board inside each I-DHU will be the same.In addition, an instrument specific data processing softwarewill be run on each I-DHU, implementing e.g. the abovementioned trigger algorithms and event detection codes.

The computational load on the I-DHU is relatively low,which allows us to use a much simpler, off-the-shelf proces-sor with flight heritage on the Processor Board with the loadbeing easily sustainable with still a large margin. The tasksof the I-DHUs can be separated into three main categories,namely data processing, instrument controlling and powerdistribution. In order to acquire/process the scientific data,the I-DHUs will:

THESEUS 39

Fig. 39 Left: the IRT focal plane division. The blue area (10×10 arcmin) is used for imaging and low resolution spectra. The orange area (5×5 ar-cmin) is used for high-resolution slit-less spectra. The size of the high-resolution spectral area is limited by the satellite pointing capabilities. Right:Teledyne Hawaii 2RG detector at ESA Payload Technology Validation section during the tests for the NISP instrument on board the EUCLID mis-sion (Credits ESA).

– collect, process and store the data stream of the respec-tive instrument;

– implement the burst trigger algorithm on SXI and XGISdata;

– implement the IRT burst follow up observation.

The I-DHU consists of two main boards that are mountedinside an aluminum case. In addition, each board exists in acold redundant (identical, non operating) version inside thebox, that can replace the nominal board in case of a failure.Switching between the nominal and redundant chain is donefrom ground via a dedicated flight-proven circuitry.

At the heart of the I-DHU design is the Processor Board.It hosts the central CPU, the mass memory, time synchro-nization and distribution circuits and the HK/health moni-toring acquisition chain. The Processor Board is connectedto the spacecraft via one SpaceWire link through which itwill receive the telecommands (TC) and send the scienceand HK/health data. On the other hand, the I-DHU is con-nected via another Spacewire link to the respective instru-ment to relay the TCs and acquire the science and HK data.The SpaceWire interface communication is handled directlythrough the main CPU (description see below), via its ex-isting dedicated hardware interfaces. The CPU is running anRTEMS (Real Time Executive Management System), an op-eration system (OS) on which the individual software tasks(detailed description below) of this I-DHU will be run inparallel. Dedicated circuitry is foreseen on the ProcessorBoard for the collection, digitization and organization of HKand health data from various voltage, current and temper-ature sensors. This will reduce the HK tasks on the mainCPU, leaving only the science meta data (like rate meters,counters) and dedicated instrument data processing to bedone there. The Processor Board will be developed by theIAAT in Tubingen, Germany. The Power Board within the I-DHU will be developed by the Centrum Badan Kosmicznych,Poland. It will generate the voltages for the Processor Boardand distribute the power to the instrument.

The main functions of the I-DHU on-board software are:instrument control, health monitoring and science data pro-

cessing, formatting. The software will be designed in orderto allow the instrument to have the complex functionalitythat it requires to allow itself to be updated and work aroundproblems automatically and with input from the ground. Therewill be a common software that is the same for all I-DHUsand an extended software part with modules specific for agiven instrument. An example of a software module com-mon to all I-DHUs is the determination of the location of aburst or transient event. The time and location of the tran-sient will be transmitted to ground using the on-board VHFsystem in a <1 kbit message. The design of the trigger soft-ware benefits from the heritage of the SVOM mission con-cept as well as past team experience on similar systems onBeppoSAX, HETE-2, AGILE as well as the INTEGRALburst alert system.

Instrument control will be possible through the softwarevia telecommands from the ground (e.g., power on & off

for individual units, loading parameters for processing &on-board calibration, investigations) and autonomously on-board (e.g., mode switching, FDIR and diagnostic data col-lection). The software will implement the standard ECSS-style PUS service telecommand packets for housekeeping,memory maintenance, monitoring etc. and some standardtelemetry packets for command acceptance, housekeeping,event reporting, memory management, time management,science data etc. The software will be able to send setupinformation to the instrument and receive and process thehousekeeping data coming back and organize these data in aconfigurable way for a lower rate transmission to the ground.Figure 40 shows the commandable operational modes man-aged by the I-DHU.

4.5 The Trigger Broadcasting Unit (TBU)

In case of trigger event it is necessary to provide the triggerdata to ground in a short time. It is expected triggered eventsoccurred at the rate of one event per orbit and the data to besent to ground is very low: the amount is <1 kb/trigger event.To support the prompt transmission of such event data to

40 Amati et al.

Fig. 40 Overview of the I-DHU operation modes and transitions.

ground segment it is necessary a link with Earth independentfrom the satellite TT&C, which can have a link with groundstation only once per orbit. The solutions evaluated for anindependent and prompt burst position broadcast to groundare by the means of:

– VHF equatorial network (SVOM and HETE-2);– Orbcom satellite network (implemented in Agile satel-

lite);– Iridium satellite network (tested by INAF/IASF on baloon

and ready to be tested in FEES/IOD n-sat);– TDRSS link.

The preferred solution is the well proven VHF broacast-ing based in the utilization of the SVOM equatorial net-work. Link with SVOM ground segment will be made of 40ground stations located around the Earth inside a ±30 degstrip. The satellite to SVOM network link shall be carryout by the Trigger Broadcasting Unit (TBU, see Fig. 41)proposed in the baseline as a unit of the Payload Module.The antennas are miniaturised for a better accommodationon PLM and SVM. The VHF ground network is the sameof SVOM mission, extensively described in the ground seg-ment section. For the definition of the VHF frequency rangea possible choice, according to ITU Article 5, could be 137-137.175 MHz, reserved to space research. This band sub-system for trigger transmission is consequently proposed forcompatibility with SVOM ground segment.

5 Satellite configuration and mission profile

The satellite configuration and design take into account amodular approach. The spacecraft platform is divided in twomodules, the Payload Module (PLM) and Service Module(SVM). The Payload Module will mechanically support theInstruments DUs (SXI, XGIS and IRT) and will host inter-nally the Instruments ICUs. The instruments SXI and XGISDUs are accommodated externally on the Payload Module

Fig. 41 General block diagram of the Trigger Broadcast Unit.

to which they are connected by means of a structural pedestals.The Instrument DUs mechanical fixing to this structure willbe designed in order to guarantee thermo-structural decou-pling from the rest of satellite. The Service Module containsall the platform subsystems and provides the mechanical in-terface with the Launcher. Figure 42 shows the spacecraftbaseline configuration.

The Payload Module is constructed around the IRT in-strument, which is partially embedded inside the PLM struc-ture and aligned with respect the S/C symmetry axis. Themodule top plane is the mounting base of the other instru-ments DUs (SXI and XGIS) which are distributed aroundthe IRT axis in order to minimize the satellite Moment of In-ertia (MoI) but also to support efficient load transfer from thespacecraft to the launch vehicle, respecting their accommo-dation constraints and thermal requirements. The IRT LoSis the reference of the overall payload. The telescope is pro-tected from straylight by a baffle; the profile of the baffle de-fines the position of the entrance window plane of the otherinstruments. The 4 SXI DUs are nominally mounted on theopposite side of the solar panels in order to keep them in thecoldest side of the satellite and to have the largest area ofthe observable sky when THESEUS lies between Sun andEarth. The SXI DUs positions and orientations are deter-mined in such a way that no X-rays reflected by IRT tube orany other satellite structure can enter into SXI FOV. The 3XGIS units are tilted in such a way that the FOV of the units

THESEUS 41

Fig. 42 THESEUS Satellite Baseline Configuration and Instrument suite accommodation.

partially overlap. The resulting overall FOV (i.e., that of thecombination of the 3 units) covers and center the FOV of the4 SXI modules. The proposed accommodation is shown inFig. 43. This configuration guarantees the required nominalSXI, XGIS and IRT Field of view combination (REQ-MIS-050). Solar array proposed baseline consists of modular pan-els in fixed configuration, the panels are composed of a pho-tovoltaic module and a mechanical on which photovoltaicmodule is mounted. At the level of the payload module topplane a Sun shield is mounted, in order to protect the in-struments from solar radiation. The structure of the Payloadmodule is provided of reinforcing shear panels and of aninternal cylinder for the IRT telescope and detector accom-modation. The internal cylinder has the function of struc-tural support and it provides also a thermal separation forIRT instrument from the rest of the module. In order to as-sure radiative thermal decoupling, the telescope is coveredby multi-layer insulation.

At the level of focal plane of IRT, where the Detectoris placed, a Miniature Pulse Tube Cooler (MPTC) systemis provided, acting with a heat-pipe system and a dedicatedradiator. Dedicated radiator is provided also for each of theother instruments (SXI and XIGS) on the pedestals. Everyradiator is positioned on the own support structure of the sin-gle instrument. The Service module is currently conceivedas composed by a single module accommodating all the plat-form units (with the exception of one boresight star trackerwhich is integrated on the IRT telescope tube to minimizerelative misalignment improving pointing knowledge per-formances. In addition to the platform star tracker, two startrackers hare foreseen in support to IRT instrument, to allowastrometric measurements independent from the system.

Table 8 Summary of Instrument Suite temperatures.

Instrument Element Operative range (C) CoolingSXI- structure/optics (-20, +20) passiveSXI- detectors -65 activeXGIS-detectors (-20, +10) passiveIRT-structure -30 activeIRT-optics -83 active

The AOCS subsystem will provide the required attitudesto support the payload observation requirements, guarantee-ing:

– the satellite agility with:– fast slewing (.60 deg/10 min) for IRT LoS pointing

to the direction of GRBs and other transient of inter-est for low resolution spectra;

– fine slewing inside the IRT full FoV to achieve theIRT LoS fine pointing necessary for IR high resolu-tion spectra.

– the satellite pointing requirements are those of REQ-MIS-060.

The subsystem is made up by software, running on the onboard computer, and by a set of sensors and actuators. ACSsoftware is organised in operative stati, each of them ded-icated to the execution of a different mission task. The 3-axis stabilized attitude control is based on a set of 4 reactionwheels used in zero momentum mode and 3 magnetic tor-quers mainly aimed to perform wheels desaturation. Sensorsused to reach the required attitude knowledge consist in starsensors and fine rate sensors; in addition magnetometers anda set of solar sensors are available to cover all the missionneeds.

42 Amati et al.

Fig. 43 THESEUS accommodation within VEGA C fairing.

The algorithms implemented in the ACS SW will pro-cess the sensors data in order to apply a proper control ableto guarantee the required pointing accuracy and stability.

The THESEUS Telemetry and Telecommand subsystemincludes a X-band TM/TC link between S/C and Earth forstandard S/C operations in charge of the RF links with groundstations with Uplinks for satellite telecommands (TC) andDownlinks for satellite telemetries (TM) and the rangingfunction to allow range measurements. During downlink op-erations, stored data, read and formatted within the PDHU,are transmitted towards the X-Band transmission assembly,where modulation, up conversion to X-Band and power am-plification will be executed. The X-Band antenna assemblywill provide for transmission to the receiving ground station.

The Thermal Control System (TCS) of the platform canbe implemented following a standard approach. It is mainlyof passive type, based on the capability of dissipating the in-ternal generated thermal power through radiators, where theheat is conducted and emitted to deep space. As general ther-mal control philosophy, all surfaces thermo-optical prop-erties are controlled by means of paints, multi layer blan-kets and/or materials with well-known characteristics. Highemissivity coatings (black paint) are used for the spacecraftinterior in order to uniformly distribute the generated powerand to avoid hot spots. Lateral closing panels having thefunction of radiators are covered with adequate coatings (e.g.white paint) to optimize the heat rejection to the space. TheTCS service to Instrument Suite will consider the temper-atures summarized in Table 8. Detailed thermal and orbitalanalyses will optimize the thermal design. THESEUS satel-lite will operate in a low equatorial orbit (altitude <600 km,inclination <5 deg). This orbital configuration will guaran-tee a low and stable background level in the high-energy

instruments. The mission has been evaluated assuming asbaseline a launch with Vega-C.

The THESEUS satellite will be equipped with a suiteof three instruments: SXI, XGIS and IRT. In summary theTHESEUS satellite will be capable to:

– to monitor a large sky sector for detecting, identifyingand localizing likely transients/burst in the SXI and XGISFOV;

– of promptly (within a few tens of seconds at most) trans-mitting to ground the trigger time and position of GRBsand other transients of interest;

– of autonomous (via SXI, XGIS or IRT trigger) orienta-tion in the sky direction of interest;

– to perform long observation of the sky direction of inter-est.

THESEUS requires a 3-axis stabilized attitude. During itsorbital period THESEUS will have distinct operational modes:

– Survey (burst hunting) mode: relevant to normal oper-ation when SXI and XGIS are searching for transients.The accessible sky for this kind of operation will be de-termined by the requirement that:

– THESEUS will have a Field of Regards (FoR) defin-ing the fraction of sky which can be monitored of64%.

– When monitoring the sky in normal operations, thenumber of re-pointings per orbit will be of the or-der of 3, resulting in observations with the SXI andXGIS of about the whole FoR every 3 orbits.

– Burst mode: after detecting a GRB or other transient ofinterest, the satellite is triggered to this mode by SXIand/or XGIS which transmit to satellite computer thequaternion of the area of interest. The satellite will au-tonomously fast repoint to place the transient within thefield of view of the IRT according to the following steps:fast slewing (.60 deg/10 min) for IRT LoS pointing tothe direction of GRBs and other transient of interest forlow resolution spectra, satellite stabilization (RPE) withinless than 0.5 arcsec fast data link for GRB coordinatecommunication to ground within a few min.

– Follow-up mode: the IRT shall observes inside the fullFoV of 10×10 arcmin the target with the pre-scriptedimaging spectroscopy sequence. In case of IRT high res-olution spectra acquisition a further satellite fine slewing(based on IRT source localization) shall be activated toplace the IRT LoS inside a reduced FoV of 5×5 arcmin.XGIS and SXI are in specific follow-up data acquisi-tion mode. In this mode the GRB observation shall beperformed in a time of 30 min. After completion of thetransient observation, THESEUS will return to SurveyMode to monitor the sky.

– IRT observatory mode: the IRT may be used as an obser-vatory for pre-selected targets through a GO programme,

THESEUS 43

Fig. 44 THESEUS Orbit Configuration (Left) and THESEUS Orbit Ground Track (Right).

driving the pointing of the satellite. XGIS and SXI areobserving as in survey mode, with the possibility of trig-gering the burst mode

No specific orbit parameter change is required during themission lifetime. THESEUS mission can be supported asbaseline by a dedicated ground station located in Malindi(3 deg S, 40 deg E). Another ground station, located in Al-cantara (2 deg S, 4 deg W), is supposed as possible back-up in case of Brasilian participation. In conformity with theselected equatorial orbit, both stations will allow highly fre-quent accesses to the satellite by a contact per orbit. Con-tact analysis between the satellite and the ground stationshas been performed considering a minimum station eleva-tion angle over local horizon of 10 deg.

6 Scientific operations, quick-look activities and datadistribution

THESEUS being a GRB mission, it is expected that theSOC will take care of nominal routine mission observationplanning with personnel in shift 7 days a week, 365 daysper year. Starting with the list of successful proposals sub-mitted at each yearly released announcement of opportu-nity, a long-term planning schedule is created using the stan-dard science planning utilities. The core of the THESEUSobserving program is, however, expected to be dominatedby the follow-up of the detected GRB and other transientevents. Therefore, detailed planning constraints (e.g., skyvisibility, and mission resources such as power and teleme-try) will need to be identified by the SOC on a daily timescale in order to shape the short term observational schedule.Payload operations exclusion windows, on-board resourcesenvelopes for payload operations and final detailed checksagainst mission, environmental and resource constraints willhave to be defined by the MOC on a similar timescale. Thebasic Mission Planning approach for all the routine scienceoperations phases will be built on the experience of previ-ous missions, such as XMM-Newton, Herschel and INTE-GRAL. The rapid rescheduling imposed by the GRB nature

of the mission will take advantage of the heritage gainedthrough the Swift mission. The science observation plan willbe modified for Target of Opportunities (ToOs) coming pri-marily from the XGIS and SXI transient event detections,but additional triggers can also be raised by other observato-ries at any wavelength. The mission will be designed to au-tomatically repoint all classes of selected events detected bythe SXI or the XGIS within minutes from the trigger. Longerreaction times are expected for external triggers. For non-GRB targets, a significant part of the traditional ToOs willbe pre-proposed as part of the calls for observing propos-als (i.e., Announcement of Opportunities, AOs). The pro-posal will include trigger criteria, plus planning informationlike coordinates, exposure times, instruments observationalmodes, etc. Also, ToO proposals can be made to the SOCfor an observation of a target outside the AO process us-ing the so-called Directors Discretionary Time (DDT). It isexpected that the THESEUS Launch and Early OperationsPhase (LEOP) will be concluded with the successful injec-tion into the chosen orbit. During Commissioning Phase theinstruments will gradually be configured to operating status,with end-to-end test of functionality of systems without re-lying on the cosmic X-ray signals. The planning cycles, up-and down-link functionality will be commissioned and thebasic instrument modes exercised. Scientific commission-ing and basic calibration verification will be completed withselected targets, including pointing and sensitivity stabil-ity demonstrations. During Performance Verification Phase(PV) selected targets will be observed to instruments capa-bilities, together with ToO, burst alert and ground segmentcapability confirmation. It is anticipated that normal scienceoperations of the first Announcement of Opportunity (AO)and the Guaranteed Time Observations (GTO) targets willstart after 6 months when satisfactory performance has beendemonstrated.

During normal science operations the data from the space-craft (excluding the short alert messages broadcasted fromthe TBAS - THESEUS burst alert system - through the TBAGS- THESEUS burst alert ground segment) is received at the

44 Amati et al.

Fig. 45 Schematic view of the Sun (SAA1) and Earh (SAA2) avoid-ance angles for the expected orbit of THESEUS; the resulting Field ofRegard (FoR) is 67% of the full sky.

MOC and sent in real time to the SOC where they are au-tomatically processed into the lower level manageable FITSevent files (including all relevant auxiliary files). Higer leveldata, including a set of pre-defined scientific products to beused for quick-look activities, are produced by a consortium-led Science Data Center (SDC). Data collected during the6 months of the PV phase will be made public only afterthe validation of the SDC and the instrument operation cen-ters, who lead the performance and health verification ofthe THESEUS instrument (together with calibration activ-ities, supported by the SOC). All THESEUS data will bemade promptly accessible to the community on a centralizedarchive (THESEUS science data archive, TSDA) as soon asthey have been processed. The Swift and Fermi experiencedemonstrated that this strategy helps in maximizing the sci-entific usage of the data, and is anyway mandated by the ur-gency of time-domain astrophysics. The centralized archive,to be hosted at SOC, will thus the primary repository for thescience data products of all levels, and will be used as thecentral “hub” during all phases of the mission. It will alsoguarantee the long term exploitability of the mission her-itage. The alerts provided by the THESEUS on-board detec-tion and localization system will be monitored and analyzedby a dedicated alert center, but also distributed to the inter-ested community immediately after the PV phase. Relevantevents discovered in the THESEUS data will be eventuallycommunicated by the SDC to the community through As-tronomer Telegrams or GCNs.

Acknowledgements S.E. acknowledges the financial support from con-tracts ASI-INAF I/009/10/0, NARO15 ASI-INAF I/037/12/0 and ASI2015-046-R.0. R.H. acknowledges GA CR grant 13-33324S. S.V. re-search leading to these results has received funding from the Euro-pean Union’s Seventh Framework Programme for research, technolog-ical development and demonstration under grant agreement no 606176.D.S. was supported by the Czech grant 16-01116S GA CR.

Complete author list and affiliations

C. Adami, Aix-Marseille Univ., CNRS, LAM, Laboratoire dAstro-physique de Marseille, 13388 Marseille, France;L. Amati, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;A. Antonelli, ASDC, Via del Politecnico snc - 00133 Rome, Italy;A. Argan, INAF-IAPS-Roma via Fosso del Cavaliere, 100, 00133,Rome, Italy;J.-L. Atteia, IRAP, Universite de Toulouse, CNRS, UPS, CNES, Toulouse,France;P. Attina, GP Advanced Projects, Italy;N. Auricchio, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;Z. Bagoly, Eotvos University, Budapest, Hungary;L. Balazs, MTA CSFK Konkoly Observatory, Konkoly-Thege M. ut13-17, Budapest, 1121, Hungary;G. Baldazzi, INFN - Sezione di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy; Department of Physics, University of Bologna,Viale Berti Pichat 6/2, I-40127 Bologna, Italy;S. Basa, Aix-Marseille Univ., CNRS, LAM, Laboratoire dAstrophysiquede Marseille, 13388 Marseille, France;R. Basak, The Oskar Klein Centre for Cosmoparticle Physics, Al-baNova, SE-106 91 Stockholm, Sweden; Department of Physics, KTHRoyal Institute of Technology, AlbaNova University Center, SE-10691 Stockholm, Sweden;P. Bellutti, FBK, via Sommarive, 18, 38123 Povo, Trento, Italy;M. G. Bernardini, INAF - Osservatorio astronomico di Brera, Via E.Bianchi 46, Merate (LC), I-23807, Italy;G. Bertuccio, Politecnico di Milano, Via Anzani 42, I-22100, Como,Italy; INFN Milano, Via Celoria 16, I-20133, Milano, Italy;F. Bianco, Center for Cosmology and Particle Physics, New York Uni-versity, 4 Washington Place, New York, NY 10003, USA;A. Blain, Department of Physics and Astronomy, University of Leices-ter, Leicester LE1 7RH, UK;S. Boci, Department of Physics, University of Tirana, Tirana, Albania;M. Boer, ARTEMIS, CNRS UMR 5270, Universite Cote d’Azur, Ob-servatoire de la Cote d’Azur, boulevard de l’Observatoire, CS 34229,F-06304 Nice Cedex 04, France;M. T. Botticella, INAF - Capodimonte Astronomical observatory Naples,Via Moiariello 16 I-80131, Naples, Italy;E. Bozzo, Department of Astronomy, University of Geneva, ch. d’Ecogia16, CH-1290 Versoix, Switzerland;O. Boulade, IRFU/Departement d’Astrophysique, CEA, Universite Paris-Saclay, F-91191, Gif-sur-Yvette, France;J. Braga, INPE, Av. dos Astronautas 1758, 12227-010, S.J.Campos-SP, Brazil;M. Branchesi, Universit degli Studi di Urbino Carlo Bo, via A. Saffi

2, 61029, Urbino; INFN, Sezione di Firenze, via G. Sansone 1, 50019,Sesto Fiorentino, Italy;S. Brandt, DTU Space - National Space Institute Elektrovej, Building327, DK-2800 Kongens Lyngby, Denmark;M. Briggs, Center for Space Plasma and Aeronomic Research, Uni-versity of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL35805, USA;E. Brocato, INAF - Astronomico di Teramo, Mentore Maggini s.n.c.,64100 Teramo, Italy;C. Budtz-Jorgensen, DTU Space - National Space Institute Elektro-vej, Building 327, DK-2800 Kongens Lyngby, Denmark;A. Bulgarelli, INAF/IASF - Bologna, Via Gobetti 101, I-40129 Bologna,Italy;L. Burderi, Dipartimento di Fisica, Universita degli Studi di Cagliari,SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy;C. Butler, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;P. Callanan, Department of Physics, University College Cork, Ireland;

THESEUS 45

Fig. 46 The main THESEUS ground station Malindi is indicated, together with a possible sub-set of the SVOM VHF ground antennas needed toreceive and broadcast the THESEUS alert messages generated on-board.

J. Camp, Astrophysics Science Division, Goddard Space Flight Cen-ter, Greenbelt, Md 20771;R. Campana, INAF/IASF-Bologna, via Piero Gobetti 101, I-40129,Bologna, Italy;S. Campana, INAF - Osservatorio astronomico di Brera, Via E. Bianchi46, Merate (LC), I-23807, Italy;E. Campolongo, OHB-Italia, Via Gallarate, 150 20151 Milano, ITALY;F. Capitanio, INAF-IAPS-Roma via Fosso del Cavaliere, 100, 00133,Rome, Italy;S. Capozziello, Dipartimento di Fisica, Universit di Napoli FedericoII, Via Cinthia, I-80126, Napoli, Italy;J. Caruana, Department of Physics, University of Malta, Msida MSD2080, Malta; Institute of Space Sciences & Astronomy, University ofMalta, Msida MSD 2080, Malta;P. Casella, INAF-Osservatorio Astronomico di Roma, Via Frascati 33,I-00040 Monte Porzio Catone, Italy;A. Castro-Tirado, IAA-CSIC, P.O. Box 03004, E-18080, Granada,Spain;B. Cenko, Astrophysics Science Division, NASA Goddard Space FlightCenter, Mail Code 661, Greenbelt, MD 20771, USA; Joint Space-ScienceInstitute, University of Maryland, College Park, MD 20742, USA;A. Celotti, SISSA, via Bonomea 265, I-34136 Trieste, Italy; INAFOs-servatorio Astronomico di Brera, via Bianchi 46, I-23807 Merate (LC),Italy; INFN - Sezione di Trieste, via Valerio 2, I-34127 Trieste, Italy;P. Chardonnet, LAPTh, Univ. de Savoie, CNRS, B.P. 110, Annecy-le-Vieux F-74941, France; National Research Nuclear University MEPhI,31 Kashirskoe Sh., Moscow 115409, Russia;Y. Chen, Institute of High Energy Physics, Beijing 100049, China;B. Ciardi, Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany;R. Ciolfi, INAF, Osservatorio Astronomico di Padova, Vicolo dell’ Os-servatorio 5, I-35122 Padova, Italy; INFN-TIFPA, Trento Institute forFundamental Physics and Applications, via Sommarive 14, I-38123Trento, Italy;S. Colafrancesco, School of Physics, University of Witwatersrand,Private Bag 3, Wits-2050, Johannesburg, South Africa;M. Colpi, Dipartimento di Fisica G. Occhialini, Universit degli Studidi Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy; INFN,Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano,Italy;A. Comastri, INAF, Osservatorio Astronomico di Bologna, Via PieroGobetti, 93/3, 40129 Bologna, Italy;V. Connaughton, Universities Space Research Association, NSSTC,320 Sparkman Drive, Huntsville, AL 35805, USA;B. Cordier, IRFU/Departement d’Astrophysique, CEA, Universite Paris-Saclay, F-91191, Gif-sur-Yvette, France;C. Contini, OHB-Italia, Via Gallarate, 150 20151 Milano, ITALY;

S. Covino, INAF-Brera Astronomical Observatory, Via Bianchi 46,23807, Merate (LC), Italy;J.-G. Cuby, LAM, Laboratoire dAstrophysique de Marseille, 13388Marseille, France;P. D’Avanzo, INAF - Osservatorio astronomico di Brera, Via E. Bianchi46, Merate (LC), I-23807, Italy;M. Dadina, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;M. G. Dainotti, Department of Physics & Astronomy, Stanford Uni-versity, Via Pueblo Mall 382, Stanford CA, 94305-4060, USA;V. D’Elia, Space Science Data Center (SSDC), Agenzia Spaziale Ital-iana, via del Politecnico, s.n.c., I-00133, Roma, Italy; INAF-OsservatorioAstronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone,Italy;A. De Luca, INAF - Istituto di Astrofisica Spaziale e Fisica CosmicaMilano, Via E. Bassini 15, I-20133 Milano, Italy;D. De Martino, INAF - Capodimonte Astronomical observatory Naples,Via Moiariello 16 I-80131, Naples, Italy;M. De Pasquale, Department of Astronomy and Space Sciences, Is-tanbul University, Beyazit, 34119, Istanbul, Turkey;E. Del Monte, INAF-IAPS-Roma via Fosso del Cavaliere, 100, 00133,Rome, Italy;M. Della Valle, INAF-Osservatorio Astronomico di Capodimonte, salitaMoiariello 16, 80131, Napoli, Italy; International Center for Relativis-tic Astrophysics, Piazzale della Repubblica 2, 65122, Pescara, Italy;A. Drago, INFN, Via Enrico Fermi 40, Frascati, Italy;Y.-W. Dong, Institute of High Energy Physics, Beijing 100049, China;G. Erdos, Wigner Research Centre for Physics, Hungarian Academyof Sciences, P.O. Box 49, H-1525 Budapest, Hungary;S. Ettori, INAF, Osservatorio Astronomico di Bologna, Via Piero Gob-etti, 93/3, 40129 Bologna, Italy; INFN, Sezione di Bologna, viale BertiPichat 6/2, 40127 Bologna, Italy;Y. Evangelista, INAF-IAPS-Roma via Fosso del Cavaliere, 100, 00133,Rome, Italy;M. Feroci, INAF-IAPS-Roma via Fosso del Cavaliere, 100, 00133,Rome, Italy;A. Ferrara, Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126Pisa, Italy; Kavli IPMU, The University of Tokyo, 5-1-5 Kashiwanoha,Kashiwa 277-8583, Japan;F. Finelli, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;M. Fiorini, INAF - Istituto di Astrofisica Spaziale e Fisica CosmicaMilano, Via E. Bassini 15, I-20133 Milano, Italy;F. Frontera, Department of Physics and Earth Sciences, University ofFerrara, Via Saragat 1, I-44122 Ferrara, Italy, and INAF-IASF, Via Go-betti, 101, I-40129 Bologna, Italy;F. Fuschino, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,

46 Amati et al.

Italy;J. Fynbo, Dark Cosmology Centre, Niels Bohr Institute, University ofCopenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark;A. Gal-Yam, Department of Particle Physics and Astrophysics, Fac-ulty of Physics, Weizmann Institute of Science, Rehovot 76100, Israel;P. Gandhi, Department of Physics & Astronomy, University of Southamp-ton, Highfield, Southampton SO17 1BJ, UK;B. Gendre, University of the Virgin Islands, 2 John Brewer’s Bay,00802 St Thomas, US Virgin Islands; Etelman Observatory, BonneResolution, St Thomas, US Virgin Islands;G. Ghirlanda, INAF - Osservatorio astronomico di Brera, Via E. Bianchi46, Merate (LC), I-23807, Italy;G. Ghisellini, INAF - Osservatorio astronomico di Brera, Via E. Bianchi46, Merate (LC), I-23807, Italy;P. Giommi, Italian Space Agency, ASI, via del Politecnico snc, 00133Roma, Italy;A. Gomboc, Centre for Astrophysics and Cosmology, University ofNova Gorica, Vipavska 11c, 5270 Ajdov scina, Slovenia;D. Gotz, IRFU/Departement d’Astrophysique, CEA, Universite Paris-Saclay, F-91191, Gif-sur-Yvette, France;A. Grado, INAF - Capodimonte Astronomical observatory Naples, ViaMoiariello 16 I-80131, Naples, Italy;J. Greiner, Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany;C. Guidorzi, Department of Physics and Earth Sciences, University ofFerrara, Via Saragat 1, I-44122 Ferrara, Italy;S. Guiriec, Department of Physics, The George Washington Univer-sity, 725 21st Street NW, Washington, DC 20052, USA; NASA God-dard Space Flight Center, Greenbelt, MD 20771, USA; Department ofAstronomy, University of Maryland, College Park, MD 20742, USA;Center for Research and Exploration in Space Science and Technology(CRESST), Greenbelt, MD 20771, USA;M. Hafizi, Department of Physics, University of Tirana, Tirana, Alba-nia;L. Hanlon, Space Science Group, School of Physics, University Col-lege Dublin, Belfield, Dublin 4, Ireland;J. Harms, Universit degli Studi di Urbino Carlo Bo, I-61029 Urbino,Italy;M. Hernanz, Institute of Space Sciences (IEECCSIC), Carrer de CanMagrans s/n, E-08193 Barcelona, Spain;J. Hjorth, Dark Cosmology Centre, Niels Bohr Institute, University ofCopenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark;A. Hornstrup, DTU Space - National Space Institute Elektrovej, Build-ing 327, DK-2800 Kongens Lyngby, Denmark;R. Hudec, Czech Technical University, Faculty of Electrical Engineer-ing, Prague 16627, Czech Republic; Kazan Federal University, Kazan420008, Russian Federations;I. Hutchinson, Department of Physics and Astronomy, University ofLeicester, Leicester LE1 7RH, UK;G. Israel, INAF-Osservatorio Astronomico di Roma, Via Frascati 33,I-00040 Monte Porzio Catone, Italy;L. Izzo, Instituto de Astrofisica de Andalucia (IAA-CSIC), Glorieta dela Astronomia s/n, 18008 Granada, Spain;P. Jonker, SRON, Netherlands Institute for Space Research, Sorbon-nelaan 2, NL-3584 CA Utrecht, The Netherlands; Department of As-trophysics/IMAPP, Radboud University, P.O. Box 9010, NL-6500 GLNijmegen, The Netherlands;Y. Kaneko, Faculty of Engineering and Natural Sciences, Sabanc Uni-versity, Orhanl Tuzla, Istanbul 34956, Turkey;N. Kawai, Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551;L. Kiss, Konkoly Observatory, Research Centre for Astronomy andEarth Sciences, Hungarian Academy of Sciences, Konkoly Thege Miklosut 15-17, H-1121 Budapest, Hungary;K. Wiersema, Department of Physics and Astronomy, University ofLeicester, Leicester LE1 7RH, UK ;

S. Korpela, University of Helsinki, Department of Physics, P.O.Box48 FIN-00014 University of Helsinki, Finland;P. Kumar, Department of Astronomy, University of Texas at Austin,Austin, TX 78712, USA;I. Kuvvetli, DTU Space - National Space Institute Elektrovej, Build-ing 327, DK-2800 Kongens Lyngby, Denmark;C. Labanti, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;M. Lavagna, Politecnico di Milano, Via La Masa 1, 20156 Milano,Italy;V. Lebrun, LAM, Laboratoire dAstrophysique de Marseille, 13388Marseille, France;E. Le Floch, IRFU/Departement d’Astrophysique, CEA, UniversiteParis-Saclay, F-91191, Gif-sur-Yvette, France;T. Li, Department of Engineering Physics and Center for Astrophysics,Tsinghua University, Beijing, China;F. Longo, Department of Physics, University of Trieste, via Valerio 2,Trieste, Italy; INFN Trieste, via Valerio 2, Trieste, Italy;F. Lu, Institute of High Energy Physics, Beijing 100049, China;M. Lyutikov, Department of Physics, Purdue University, 525 North-western Avenue, West Lafayette, IN 47907-2036 and Department ofPhysics and McGill Space Institute, McGill University, 3600; Univer-sity Street, Montreal, Quebec H3A 2T8, Canada;A. MacFadyen, Center for Cosmology and Particle Physics, New YorkUniversity, New York, NY, USA;U. Maio, Leibniz Institut for Astrophysics, An der Sternwarte 16, 14482Potsdam, Germany; INAF-Osservatorio Astronomico di Trieste, viaG. Tiepolo 11, 34131 Trieste, Italy;E. Maiorano, INAF-IASF Bologna, via Piero Gobetti 101, I-40129Bologna, Italy;G. Malaguti, INAF-IASF Bologna, via Piero Gobetti 101, I-40129Bologna, Italy;P. Malcovati, Department of Electrical, Computer, and BiomedicalEngineering, University of Pavia, Pavia, Italy;D. Malesani, Dark Cosmology Centre, Niels Bohr Institute, Universityof Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Den-mark;L. Maraschi, INAF - Osservatorio astronomico di Brera, Via E. Bianchi46, Merate (LC), I-23807, Italy;R. Margutti, Center for Interdisciplinary Exploration and Research inAstrophysics (CIERA) and Department of Physics and Astrophysics,Northwestern University, Evanston, IL 60208, USA;M. Marisaldi, INAF-IASF Bologna, via Piero Gobetti 101, I-40129Bologna, Italy;A. Martin-Carrillo, Space Science Group, School of Physics, Univer-sity College Dublin, Belfield, Dublin 4, Ireland;N. Masetti, INAF-IASF Bologna, via Piero Gobetti, 101, I-40129 Bologna,Italy; Departamento de Ciencias Fısicas, Universidad Andres Bello,Fernandez Concha 700, Las Condes, Santiago, Chile;S. McBreen, School of Physics, University College Dublin, Belfield,Stillorgan Road, Dublin 4, Ireland;A. Melandri, INAF - Osservatorio Astronomico di Brera, via E. Bianchi36, I-23807 Merate (LC), Italy;S. Mereghetti, INAF - IASF Milano, Via E. Bassini 15, 20133 Milano,Italy;R. Mignani, INAF - Istituto di Astrofisica Spaziale e Fisica CosmicaMilano, via E. Bassini 15, 20133, Milano, Italy; Janusz Gil Institute ofAstronomy, University of Zielona Gora, Lubuska 2, 65-265, ZielonaGora, Poland;M. Modjaz, Center for Cosmology and Particle Physics, Departmentof Physics, New York University, 726 Broadway office 1044, NewYork, NY 10003, USA ;G. Morgante, INAF-IASF Bologna, via Piero Gobetti, 101, I-40129Bologna, Italy;B. Morelli, OHB-Italia, Via Gallarate, 150 20151 Milano, ITALY;D. Morris, Etelman Observatory, St. Thomas, United States Virgin Is-

THESEUS 47

lands 00802, USA; College of Science and Math, University of VirginIslands, St. Thomas, United States Virgin Islands 00802, USA;C. Mundell, Department of Physics, University of Bath, ClavertonDown, Bath BA2 7AY, UK;H. U. Nargaard-Nielsen, DTU Space - National Space Institute Elek-trovej, Building 327, DK-2800 Kongens Lyngby, Denmark;S. Nagataki, Astrophysical Big Bang Laboratory (ABBL), RIKEN,Saitama 351-0198, Japan;L. Nicastro, INAF - Instituto di Astrofisica Spaziale e Fisica Cosmica,Via Piero Gobetti 101, I-40129 Bologna, Italy;P. O’Brien, Department of Physics and Astronomy, University of Le-icester, Leicester LE1 7RH, UK;N. Omodei, W. W. Hansen Experimental Physics Laboratory, Kavli In-stitute for Particle Astrophysics and Cosmology, Department of Physicsand SLAC National Accelerator Laboratory, Stanford University, Stan-ford, CA 94305, USA;M. Orlandini, INAF-IASF Bologna, via Piero Gobetti, 101, I-40129Bologna, Italy;P. Orleanski, Space Research Center of the Polish Academy of Sci-ences, Warsaw, Poland;J. P. Osborne, Department of Physics and Astronomy, University ofLeicester, Leicester LE1 7RH, UK;A. Paizis, INAF - IASF Milano, Via E. Bassini 15, 20133 Milano,Italy;E. Palazzi, INAF-IASF Bologna, via Piero Gobetti, 101, I-40129 Bologna,Italy;S. Paltani, Department of Astronomy, University of Geneva, ch. d’Ecogia16, CH-1290 Versoix, Switzerland;F. Panessa, INAF-IAPS-Roma via Fosso del Cavaliere, 100, 00133,Rome, Italy;G. Pareschi, INAF - Osservatorio astronomico di Brera, Via E. Bianchi46, Merate (LC), I-23807, Italy;P. Pata, Department of Radioelectronics, Faculty of Electrical Engi-neering, Czech Technical University in Prague, Technicka 2, 166 27Prague 6, Czech Republic;A. Pe’er, Department of Physics, University College Cork, Ireland;A. V. Penacchioni, ICRA Net, Piazza della Repubblica 10, I-65122Pescara, Italy; ASI Science Data Center, Via del Politecnico s.n.c., I-00133 Rome, Italy; Department of Physical Sciences, Earth and Envi-ronment, University of Siena, Via Roma 56, I-53100 Siena, Italy;V. Petrosian, W. W. Hansen Experimental Physics Laboratory, KavliInstitute for Particle Astrophysics and Cosmology, Department of Physicsand SLAC National Accelerator Laboratory, Stanford University, Stan-ford, CA 94305, USA;E. Pian, Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126Pisa, Italy;E. Piedipalumbo, Dipartimento di Fisica, Universita degli Studi diNapoli Federico II, Compl. Univ. Monte S. Angelo, 80126 Naples,Italy; INFN, Sez. di Napoli, Compl. Univ. Monte S. Angelo, Edificio6, via Cinthia, 80126 Napoli, Italy;T. Piran, Racah Institute of Physics, The Hebrew University of Jerusalem,Jerusalem 91904, Israel;P. Piranomonte, INAF-Osservatorio Astronomico di Roma, Via Fras-cati 33, I-00040 Monte Porzio Catone, Italy;L. Piro, INAF-IAPS-Roma via Fosso del Cavaliere, 100, 00133, Rome,Italy;A. Rachevski, INFN - Sezione di Trieste, via Valerio 2, I-34127 Tri-este, Italy;G. Rauw, Universite de Liege, Quartier Agora, Allee du 6 Aout 19c,B-4000 Sart Tilman, Liege, Belgium;M. Razzano, Department of Physics, University of Pisa and INFN-Pisa,Pisa, I-56127;A. Read, Department of Physics and Astronomy, University of Leices-ter, Leicester LE1 7RH, UK;V. Reglero, Image Processing Laboratory, University of Valencia C/CatedraticoJose Beltran, 2, 46980 Paterna (Valencia), Spain;

E. Renotte, Centre Spatial de Liege, Parc Scientifique du Sart TilmanAvenue du Pre-Aily, 4031 Angleur-Liege, Belgium;L. Rezzolla, Institut fur Theoretische Physik, Johann Wolfgang Goethe-Universitat, Max-von-Laue-Straße 1, 60438 Frankfurt, Germany; Frank-furt Institute for Advanced Studies, Ruth-Moufang-Straße 1, 60438Frankfurt, Germany;J. Rhoads, School of Earth and Space Exploration, Arizona State Uni-versity, Tempe, AZ 85287, USA; Astrophysics Science Division, God-dard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771,USA;T. Rodic, SPACE-SI, Slovenian Centre of Excellence for Space Sci-ences and Technologies, Ljubljana, Slovenia;P. Romano, INAF-Osservatorio Astronomico di Brera, via E. Bianchi,46. I-23807 Merate, Italy;P. Rosati, Universita degli Studi di Ferrara, Via Saragat 1, Ferrara,Italy;A. Rossi, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;R. Ruffini, ICRANet, P.zza della Repubblica 10, 65122 Pescara, Italy;ICRA and Dipartimento di Fisica, Sapienza Universita di Roma, P.leAldo Moro 5, 00185 Rome, Italy;F. Ryde, The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova,SE-106 91 Stockholm, Sweden; Department of Physics, KTH RoyalInstitute of Technology, AlbaNova University Center, SE-106 91 Stock-holm, Sweden;L. Sabau-Graziati, Division de Ciencias del Espacio (INTA), Torre-jon de Ardoz, Madrid, Spain;R. Salvaterra, INAF - Istituto di Astrofisica Spaziale e Fisica CosmicaMilano, via E. Bassini 15, 20133, Milano, Italy ;A. Santangelo, Institut fur Astronomie und Astrophysik, AbteilungHochenergieastrophysik, Kepler Center for Astro and Particle Physics,Eberhard Karls Universitat, Sand 1, D 72076 Tubingen, Germany;S. Savaglio, Physics Dept., University of Calabria, via P. Bucci, 87036,Arcavacata di Rende, Italy;V. Sguera, INAF-IASF Bologna, via P. Gobetti, 101. I-40129 Bologna,Italy;P. Schady, Max Planck Institute for extraterrestrial Physics, Giessen-bachstrasse 1, 85748 Garching, Germany;M. Sims, Department of Physics and Astronomy, University of Leices-ter, Leicester LE1 7RH, UK;W. Skidmore, Warren Skidmore, Thirty Meter Telescope InternationalObservatory, 100 W. Walnut St., Suite 300, Pasadena, CA 91124, USA;L. Song, Key Laboratory of Particle Astrophysics, Institute of HighEnergy Physics, Chinese Academy of Sciences, Beijing 100049, China;J. Soomin, Instituto de Astrofisica de Andalucia (IAA-CSIC), Glorietade la Astronomia s/n, 18008 Granada, Spain;E. Stanway, Department of Physics, University of Warwick, GibbetHill Road, Coventry, CV4 7AL, UK;R. Starling, University of Leicester, Department of Physics and As-tronomy and Leicester Institute of Space and Earth Observation, Uni-versity Road, Leicester LE1 7RH, UK;G. Stratta, Universita degli Studi di Urbino Carlo Bo, I-61029 Urbino,Italy;D. Szecsi, Astronomical Institute of the Czech Academy of Sciences,Fricova 298, 25165 Ondrejov, Czech Republic; School of Physics andAstronomy and Institute of Gravitational Wave Astronomy, Universityof Birmingham, Edgbaston, Birmingham B15 2TT, UK;G. Tagliaferri, INAF - Osservatorio astronomico di Brera, Via E. Bianchi46, Merate (LC), I-23807, Italy;N. Tanvir, University of Leicester, Department of Physics and Astron-omy and Leicester Institute of Space & Earth Observation, UniversityRoad, Leicester, LE1 7RH, UK;C. Tenzer, Institut fur Astronomie und Astrophysik, Abteilung Hoch-energieastrophysik, Kepler Center for Astro and Particle Physics, Eber-hard Karls Universitat, Sand 1, D 72076 Tubingen, GermanyM. Topinka, Dublin Institute for Advanced Studies, School of Cosmic

48 Amati et al.

Physics, 31 Fitzwilliam Place, Dublin 2, Ireland;E. Troja, Department of Astronomy, University of Maryland, CollegePark, Maryland 20742-4111, USA; NASA Goddard Space Flight Cen-ter, 8800 Greenbelt Rd, Greenbelt, Maryland 20771, USA;Y. Urata, Institute of Astronomy, National Central University, Chung-Li 32054, TaiwanM. Uslenghi, INAF-IASF, via E. Bassini 15, 20133 Milano, Italy;A. Vacchi, INFN Trieste, via Valerio 2, Trieste, Italy;L. Valenziano, INAF/IASF Bologna, via Gobetti 101, Bologna, Italy;M. van Putten, Sejong University, 98 Gunja-Dong Gwangin-gu, Seoul143-747, Korea;E. Vanzella, INAF Osservatorio Astronomico di Bologna, via Ranzani1, 40127 Bologna, Italy;S. Vercellone, INAF-Osservatorio Astronomico di Brera, via E. Bianchi,46. I-23807 Merate, Italy;S. Vergani, GEPI, Observatoire de Paris, PSL Research University,CNRS, Place Jules Janssen, 92190 Meudon, France; INAF/OsservatorioAstronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy;G. Vianello, SLAC National Accelerator Laboratory, Stanford Univer-sity, Stanford, CA 94305, USA;S. Vinciguerra, Institute of Gravitational Wave Astronomy & Schoolof Physics and Astronomy, University of Birmingham, Birmingham,B15 2TT, United Kingdom;D. Watson, Dark Cosmology Centre, Niels Bohr Institute, Universityof Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Den-mark;R. Willingale, Department of Physics and Astronomy, University ofLeicester, Leicester LE1 7RH, UKC. Wilson-Hodge, NASA/Marshall Space Flight Center, Huntsville,AL, USA;S. Vojtech, Astronomical Institute Academy of Sciences of the CzechRepublic, Fricova 1, Ondrejov, CZ-25165, Czech Republic;D. Yonetoku, Faculty of Mathematics and Physics, Kanazawa Univer-sity, Ishikawa 920-1192, Japan;G. Zampa, INFN Trieste, via Valerio 2, Trieste, Italy;N. Zampa, INFN Trieste, via Valerio 2, Trieste, Italy;B. Zhang, Department of Physics and Astronomy, University of Nevada,Las Vegas, NV 89154, USA;B. B. Zhang, IAA-CSIC, P.O. Box 03004, E-18080, Granada, Spain;S. Zhang, Institute of High Energy Physics, Beijing 100049, China;S.-N. Zhang, Institute of High Energy Physics, Beijing 100049, China;J. Zicha, Department of Instrumentation and Control Engineering, Fac-ulty of Mechanical Engineering, Czech Technical University in Prague,Technicka 4, 166 07 Praha 6, Czech Republic

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