Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1 FIRST RESULTS ON THE HIGH FIRST RESULTS ON THE HIGH ENERGY COSMIC RAY ENERGY COSMIC RAY ELECTRON SPECTRUM FROM ELECTRON SPECTRUM FROM FERMI LAT FERMI LAT Alexander Moiseev CRESST/NASA GSFC and University of Maryland for the Fermi LAT Collaboration
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Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1 FIRST RESULTS ON THE HIGH ENERGY COSMIC RAY ELECTRON SPECTRUM FROM FERMI LAT Alexander Moiseev.
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Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 11
FIRST RESULTS ON THE FIRST RESULTS ON THE HIGH ENERGY COSMIC HIGH ENERGY COSMIC
RAY ELECTRON RAY ELECTRON SPECTRUM FROM SPECTRUM FROM
FERMI LAT FERMI LAT
Alexander Moiseev
CRESST/NASA GSFC and University of Maryland
for the Fermi LAT Collaboration
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 22
2008: New results on high energy 2008: New results on high energy cosmic ray electrons and positronscosmic ray electrons and positrons
Astrophysicists are excited: •Spectral feature at ~ 620 GeV reported by ATICATIC and PPB-BETSPPB-BETS suggests a nearby source (astrophysical or exotic)• Pamela :Pamela : increase of positron fraction above 10 GeV also suggests new source or production process at high energy• H.E.S.S.H.E.S.S. detects spectrum steepening above ~1 TeV : local source? Weaker re-acceleration?• More than 100 papersMore than 100 papers mentioning these results within a few months
PAMELA
Electron + positron results above 100 GeV
Positron fraction
model
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 33
Fermi LAT Fermi LAT CollaborationCollaboration
United States (NASA and DOE)United States (NASA and DOE)• California State University at SonomaCalifornia State University at Sonoma• Goddard Space Flight CenterGoddard Space Flight Center• Naval Research LaboratoryNaval Research Laboratory• Ohio State UniversityOhio State University• Stanford University (HEPL, KIPAC and Stanford University (HEPL, KIPAC and
SLAC)SLAC)• University of California at Santa Cruz – University of California at Santa Cruz –
SCIPPSCIPP• University of DenverUniversity of Denver• University of WashingtonUniversity of Washington
JapanJapan• Hiroshima UniversityHiroshima University• Institute for Space and Institute for Space and Astronautical Science / JAXAAstronautical Science / JAXA• RIKEN RIKEN • Tokyo Institute of TechnologyTokyo Institute of Technology
SwedenSweden• Royal Institute of Technology Royal Institute of Technology (KTH)(KTH)• Stockholm UniversityStockholm University
122 full members
95 affiliated scientists
38 management, engineering and technical members
68 post-doctoral members
105 graduate students
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 44
Two instruments onboard Fermi:Two instruments onboard Fermi: Large Area Telescope LATLarge Area Telescope LAT
– main instrumentmain instrument, gamma-ray telescope, , gamma-ray telescope, 20 MeV - >300 GeV energy range 20 MeV - >300 GeV energy range
– scanning (main) mode - 20% of the sky all scanning (main) mode - 20% of the sky all the time; all parts of sky for ~30 min. the time; all parts of sky for ~30 min. every 3 hoursevery 3 hours
– ~ 2.4 sr field of view, 8000 cm~ 2.4 sr field of view, 8000 cm22 effective effective area above 1 GeVarea above 1 GeV
– high energy (5-10%) and spatial (~3high energy (5-10%) and spatial (~300 at at 100 MeV and <0.1100 MeV and <0.100 at 1 GeV) resolution at 1 GeV) resolution
– 1 1 μμs timing, <30 s timing, <30 μμs dead times dead time
GLAST Burst Monitor GBMGLAST Burst Monitor GBM
5-year mission (10-year goal), 565 km circular orbit, 25.60 inclination
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 55
The LAT Instrument The LAT Instrument OverviewOverview
e+
e–
Pair-conversion gamma-ray telescope: 16 identical “towers” providing conversion of γ into e+e- pair and determination of its arrival direction (Tracker) and energy (Calorimeter). Covered by segmented AntiCoincidence Detector which rejects the charged particles backgroundSilicon-stripped tracker:Silicon-stripped tracker: 18 double-plane single-side (x and y) interleaved with 3.5% X0 thick (first 12) and 18% X0 thick (next 4) tungsten converters. Strips pitch is 228 μm; total 8.8×105 readout channels
Hodoscopic CsI CalorimeterHodoscopic CsI Calorimeter Array of 1536 CsI(Tl) crystals in 8 layers.
Segmented Anticoincidence Detector:Segmented Anticoincidence Detector: 89 plastic scintillator tiles and 8 flexible scintillator ribbons. Segmentation reduces self-veto effect at high energy.
Electronics SystemElectronics System Includes flexible, robust hardware trigger and software filters.
~1.7 m
~1 m
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 66
Launch from Cape Canaveral, June 11, 2008
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 77
Being a Being a γγ–ray telescope, LAT intrinsically is an –ray telescope, LAT intrinsically is an electron spectrometer. electron spectrometer. We onlyWe only needed to teach it needed to teach it how to distinguish how to distinguish electrons from hadronselectrons from hadrons
We found that:We found that:
• Photon reconstruction works perfectly for electronsPhoton reconstruction works perfectly for electrons
• All events above ~20 GeV are downlinkedAll events above ~20 GeV are downlinked
• Residual hadron contamination below 20% is confirmedResidual hadron contamination below 20% is confirmed
• Energy range can be extended to at least 1 TeV Energy range can be extended to at least 1 TeV
FERMI FLIGHT DATA ANALYSIS FOR FERMI FLIGHT DATA ANALYSIS FOR ELECTRONSELECTRONS
Segmented calorimeter with imaging capability:
– fraction of mm to a few mm accuracy position reconstruction depending on energy
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 99
Event energy Event energy reconstructionreconstruction
Beam Energy = 20 GeV
Beam Energy = 100 GeV
Beam Energy = 280 GeV
CAL Layer
CAL Layer
CAL Layer
En
erg
yEn
erg
yEn
erg
y
BT DataMC
1. Reconstruction of the most probable value for the event energy:
- based on calibration of the response of each of 1536 calorimeter crystals
- energy reconstruction is optimized for each event
- calorimeter imaging capability is heavily used for fitting shower profile
- tested at CERN beams with LAT Calibration Unit
Very good agreement between beam test and Monte Carlo
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1010
Energy resolutionEnergy resolution
Agreement between MC and beam test within a few percent up to 280 GeV we can be confident in MC we can be confident in MC we we have reasonable grounds to extend the energy have reasonable grounds to extend the energy range to 1 TeV relying on Monte Carlo simulations range to 1 TeV relying on Monte Carlo simulations
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1.1. Goal 1Goal 1 – keep residual hadron contamination at less than 20% of the number of electrons which pass event classification, over the whole energy range
2.2. Goal 2Goal 2 – maximize the effective geometric factor for electrons
3.3. Approach Approach
- optimize and tune the selections to separate electrons from hadrons and photons (basic cuts + Classification Tree analysis ). It utilizes about 30 LAT ntuple variables from all the LAT subsystems
- use MC-generated “background” runs which contain the best known fluxes of HE charged particles on Fermi orbit in order to optimize electron selections and determine residual proton contamination
- use MC- generated uniform isotropic “all Electrons” runs in order to determine instrument response function (effective geometric factor)
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1212
Achieved electron-hadron Achieved electron-hadron separation and effective separation and effective
geometric factorgeometric factor• Candidate electrons pass on average 12.5 X0 ( Tracker and
Calorimeter added together)
• Simulated residual hadron contamination (5-21% increasing with the energy) is deducted from resulting flux of electron candidates
• Effective geometric factor exceeds 2.5 m2sr for 30 GeV to 200 GeV, and decreases to ~1 m2sr at 1 TeV
• Full power of all LAT subsystems is in use: tracker, calorimeter and ACD act together
Geometric Factor
Residual hadron
contamination
Key issue: good knowledge and confidence in Instrument Response Function
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1313
Flight event Flight event display display
Electron candidate, Electron candidate, 844 GeV844 GeV
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Validation of the flight dataValidation of the flight data
Task:Task: compare the efficiency of all “cuts” for flight data and MC events
Approach:Approach:
- Plot from the flight data the histogram of each variable involved in the electron selections, one at a time, after applying all other cuts
- check if the flight histograms match the simulated ones, and account for the differences in systematic errors for the reconstructed spectrum
Example for the variable (shower transverse size)
Analysis variables demonstrate good agreement between the flight data and MC
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1515
Assessment of systematic errorsAssessment of systematic errors
Contributors:Contributors: 1. 1. Uncertainty in geometric factor – comes from the residual discrepancy between Monte Carlo and the data. Carefully estimated for each variable used in the analysis
2. Uncertainty in determination of residual hadron contamination
- comes mostly from the uncertainty of the primary proton flux (~ 20%)
- we validated the hadronic interaction model with beam test data
Contributors 1 and 2 result in total systematic error in the spectrum ranging from 10% at low energy end to 25-30% at
high energy end (full width)
3. Possible bias in absolute energy determination 3. Possible bias in absolute energy determination
- Included separately in the resulting spectrum as (+5, -10)% - estimated from MC simulations, calorimeter calibration and CERN beam test.
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1616
Fermi-LAT electron spectrum from 20 Fermi-LAT electron spectrum from 20 GeV to 1 TeVGeV to 1 TeV
Submitted to PRL on March 19, 2009
Accepted April 21
Measurement of the Cosmic Ray e++e- Spectrum from 20 GeV to 1 TeV with the Fermi Large Area TelescopeA. A. Abdo et al. (Fermi LAT Collaboration)
Published 4 May 2009 See accompanying Viewpoint Physics 2, 37 (2009) Designed as a high-sensitivity gamma-ray observatory, the Fermi Large Area Telescope is also an electron detector with a large acceptance exceeding 2 m2 sr at 300 GeV. Building on the gamma-ray analysis, we have developed an efficient electron detection strategy which provides sufficient background rejection for measurement of the steeply falling electron spectrum up to 1 TeV. Our high precision data show that the electron spectrum falls with energy as E-3.0 and does not exhibit prominent spectral features. Interpretations in terms of a conventional diffusive model as well as a potential local extra component are briefly discussed.
Total statistics collected for 6 months of Fermi LAT observations
> 4 million electrons above 20 GeV
> 400 electrons in last energy bin (770-1000 GeV)
First public First public release on release on May 2, 2009 May 2, 2009 at April’s APS at April’s APS meeting in meeting in DenverDenver
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1717
And finally we want to check – could we And finally we want to check – could we miss “ATIC-like” spectral feature?miss “ATIC-like” spectral feature?
We validated the spectrum reconstruction by:– comparing the results for different path length subsets– varying the electron selections – simulating the LAT response to a spectrum with an “ATIC-like” feature:
This demonstrates that the Fermi LAT would have been able to reveal “ATIC-like” spectral feature with high
confidence if it were there. Energy resolution is not an issue with such a wide feature
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1818with 2 per degree of freedom of 9.7 / 23
Profumo - 1/8
(χ2 =9.7, d.o.f 24)
astro-ph 0905. 0636 (May 4, 2009)
Some interpretation…Some interpretation…
Spectrum can be fit by Diffuse Galactic Cosmic-Ray Source Model (electrons accelerated by continuously distributed astrophysical sources, likely SNR), with harder injection spectral index (-2.42) than in previous CR models (-2.54). All that within our current uncertainties, both statistical and systematic
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 1919
Fermi CRE data exacerbates the discrepancy between a
purely secondary diffuse cosmic-ray origin for positrons
and the positron fraction measured by Pamela
Now – let’s include recent Pamela result Now – let’s include recent Pamela result on positron fraction:on positron fraction:
Harder primary CRE spectrum steeper secondary-to-primary e+/e- ratio
New “conventional” CRE models
Old “conventional” CRE Model
Profumo - 2/8
S. Profumo, APS, 050409
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 2020
Need other contributors of electrons:Need other contributors of electrons:
PulsarsPulsars:: Most significant contribution to high-energy CRE:
Example of fit to both Fermi and Pamela data with known
(ATNF catalogue) nearby, mature pulsars and with a single,
nominal choice for the e+/e- injection parameters
Profumo - 4/8
S. Profumo, APS, 050409
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 2121
What if we randomly vary the pulsar parameters
relevant for e+e- production?
(injection spectrum, e+e- production efficiency, PWN “trapping” time)
Under reasonable assumptions, electron/positron emission from pulsars
offers a viable interpretation of Fermi CRE data which is
also consistent with the HESS and Pamela results. Maybe too many degrees of freedom, but the assumption is plausible
Profumo - 5/8
S. Profumo, APS, 050409
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 2222
Dark matter: the impact Dark matter: the impact
of the new Fermi CRE dataof the new Fermi CRE data
1. Much weaker rationale to postulate a DM mass in the 0.3-1 TeV range (“ATIC bump”) motivated by the CR electron+positron spectrum
2. If the Pamela positron excess is from DM annihilation or decay, Fermi CRE data set stringent constraints on such interpretation
3. Even neglecting Pamela, Fermi CRE data are useful to put limits on rates for particle DM annihilation or decay
4. We find that a DM interpretation to the Pamela positron fraction data consistent with the new Fermi-LAT CRE is a viable possibility.
Profumo - 6/8
Origin of the local source is still unclear – astrophysical or “exotic”
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H.E.S.S. astro-ph 0905.0105, May 1, 2009
• The measured spectrum is compatible with a power law within our current systematic errors. The spectral index (-3.04) is harder than expected from previous experiments and simple theoretical considerations
• “Pre-Fermi” diffusive model requires a harder electron injection spectrum (by 0.12) to fit the Fermi data, but inconsistent with positron excess reported by Pamela if it extends to higher energy
• Additional component of electron flux from local source(s) may solve the problem; its origin, astrophysical or exotic, is still unclear
• Valuable contribution to the calculation of IC component of diffuse gamma radiation
SUMMARY
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 2424
The last thing: Positron spectrum, derivedPositron spectrum, derived from FermiFermi electron + positron spectrum and PamelaPamela
positron ratio
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Future plans: Search for anisotropy in the electron flux – contributes to the understanding of the “extra” source origin
Study systematic errors in energy and instrument response to determine whether or not the observed spectral structure is significant – also critical for understanding of the source origin, as well as models constrains
Expand energy range down to ~ 5 GeV (lowest possible for Fermi orbit) and up to ~ 2 TeV, in order to reveal the spectral shape above 1 TeV
Increase the statistics at high energy end. Each year Fermi-LAT will collect ~ 400 electrons above 1 TeV with the current selections if the spectral index stays unchanged
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BACK-UPS
Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009Alexander Moiseev NASA/GSFC Pamela workshop May 11, 2009 2727
And finally we want to check – could we And finally we want to check – could we miss “ATIC-like” spectral feature?miss “ATIC-like” spectral feature?
We validated the spectrum reconstruction by:– comparing the results for different path length subsets– varying the electron selections – simulating the LAT response to a spectrum with an “ATIC-like” feature:
6 month Fermi LAT spectrum with the added spectral feature reported by ATIC, scaled for the statistics to be obtained by LAT for the same observation time:
Black points – LAT electron spectrum
Blue points: LAT spectrum + “ATIC-like” bump. LAT ΔE/E (68%, FW) = 18%. The excess would be ~ 7,000 events on the top of “background” of ~ 14,000 events between 300 and 800 GeV
Red points – the same but if LAT had twice worse energy resolutionThis demonstrates that the Fermi LAT would have been
able to reveal “ATIC-like” spectral feature with high confidence if it were there. Energy resolution is not an