Anticipated problems in our current approach to identify gamma-ray sources

Anticipated problems in our current Anticipated problems in our current approach to identify gamma-ray sourcesapproach to identify gamma-ray sources

or: How far automated source or: How far automated source identification might succeed identification might succeed

before we will strugglebefore we will struggle

Olaf Reimer & Diego F. Torres (Stanford) (LLNL)

status quo: presentations from joint status quo: presentations from joint diffuse/catalog workshop earlier this yeardiffuse/catalog workshop earlier this year

Counterpart listings / probabilityCounterpart listings / probability

Counterpart listings / probabilityCounterpart listings / probabilityWhat’s problematic here?What’s problematic here?

Let’s review some examples:Let’s review some examples:

An “average” EGRET source: 3EG J1249-8330An “average” EGRET source: 3EG J1249-8330[[9595 =0.66 ° , 2 x 10 =0.66 ° , 2 x 10-7-7 ph cm ph cm-2-2 s s-1-1]]1)1) 4 XMM-EPIC pointing -> 148 X-ray sources4 XMM-EPIC pointing -> 148 X-ray sources2)2) statistical evaluation of counterparts statistical evaluation of counterparts 3)3) does computing does computing a counterpart probability

pc = ppos x p(i)SED x p(i)

var x p(i)ext x …

will yield a source identification here ?

No, since for N = 94…148 -> pc will be numericallyundistinguishable in the systematics of its computation

Counterpart listings / probabilityCounterpart listings / probabilityWhat’s problematic here?What’s problematic here?

more examples:more examples:

An EGRET source in the galactic plane An EGRET source in the galactic plane 3EG J1824-1514 3EG J1824-1514 [~4 x 10[~4 x 10-7-7 ph cm ph cm-2-2 s s-1-1]]1)1) counterparts in existing catalogs: counterparts in existing catalogs: many already & even many already & even

more when proceeding into specialized catalogsmore when proceeding into specialized catalogs2)2) complicated region in terms of diffuse count prediction:complicated region in terms of diffuse count prediction:

comparable large uncertainty region!comparable large uncertainty region!3)3) does computing does computing a counterpart probability

pc = ppos x p(i)SED x p(i)

var x p(i)ext x …

will yield a source identification ?

No, since p(i)var is not granted beforehand (stochastic)

ppos would exclude the object (it’s the six nearest byconsidering two catalogs only)[NRAO VLA Sky Survey (NVSS) + RASS FSC]

LS5039

Counterpart listings / probabilityCounterpart listings / probabilityWhat’s problematic here?What’s problematic here?

Argument: LAT will have better source Argument: LAT will have better source locations, so here a better example locations, so here a better example from EGRETfrom EGRET

A bright EGRET source at high galactic latitudes A bright EGRET source at high galactic latitudes 3EG J1835+5918 3EG J1835+5918 [6 x 10[6 x 10-7-7 ph cm ph cm-2-2 s s-1-1!]!]1)1) counterparts in existing catalogs: counterparts in existing catalogs: ØØ2) 2) dedicated deep HRI pointing: 10 counterparts with almost similar dedicated deep HRI pointing: 10 counterparts with almost similar

characteristics -> MWL follow-upscharacteristics -> MWL follow-ups3)3) does computing does computing a counterpart probability

pc = ppos x p(i)SED x p(i)

var x p(i)ext x …

will yield a source identification ?

No, since either N = 0: pc = 0 or N > 0: ppos alone dominates pc

Counterpart listingsCounterpart listingsWhat’s problematic here?What’s problematic here?Argument: LAT will be even better than Argument: LAT will be even better than this, so here an example from VHE gamma-raysthis, so here an example from VHE gamma-rays

HESS J1303-631 (13h03m00.4sHESS J1303-631 (13h03m00.4s±4.4s±4.4s and δ=−63°11’55” and δ=−63°11’55”±31”)±31”)at least 5 catalog counterparts listed in at least 5 catalog counterparts listed in several several counterpart categoriescounterpart categories

does computing does computing a counterpart probability

pc = ppos x p(i)SED x p(i)

var x p(i)ext x …

will yield a source identification ?

No, since p(i)ext = 0 -> pc = 0 ! (point source catalogs)

But source is extended!

After visually appealing cases a more general discussion:After visually appealing cases a more general discussion:

positional coincidence probability ppos(i) -> 1 for observational coverage

(ii) existing catalogs are by definition incomplete, so a reasonable ppos

will depend on a sizable number of mwl catalogs in order to keep ppos < 1, but:

(iii) actual value of ppos will be determined from the inherent treatment of cuts/completeness/quality of each individual catalog to be considered, -> thus ppos will have an indeed different meaning for regions of

different source densities -> source locations

Almost every catalog will exhibit different quantities here, Almost every catalog will exhibit different quantities here, whichwhichwill make it impossible to define a uniform pwill make it impossible to define a uniform ppos pos over the skyover the sky

practically (if the gamma-ray observables are the same) p(A)pos ≠ p(B)pos for a single LAT source if catalogs from different

source populations A, B are involved

ppos LAT source1 ≠ ppos LAT source2 at different spatial locations

Why did it worked for EGRET ?Why did it worked for EGRET ?(“A” blazars / Mattox et al. 97,01 / Soward-Emmered et al. 03-05) (“A” blazars / Mattox et al. 97,01 / Soward-Emmered et al. 03-05)

Only the Only the dominantdominant population of high-energy gamma-ray emitters population of high-energy gamma-ray emittershas been probed (blazar-class AGN) has been probed (blazar-class AGN)

It does It does not work at allnot work at all for galactic sources, i.e. we never for galactic sources, i.e. we never proceeded from statistical evidence for SNRs (Dermer & Sturner,proceeded from statistical evidence for SNRs (Dermer & Sturner,Esposito et al., Romero et al. to individual IDs !Esposito et al., Romero et al. to individual IDs !

Why does it works in X-ray astronomy? Why does it works in X-ray astronomy?

pppospos <-> small psf in focusing X-ray telescopes <-> small psf in focusing X-ray telescopesresulting in sheer dominance of resulting in sheer dominance of pppospos (+ ability to handle source extension) (+ ability to handle source extension)

also: the apparatus is well-defined for arcsec-psf point-sourcesalso: the apparatus is well-defined for arcsec-psf point-sources(Sutherland & Saunders 1992 “On the likelihood ratio for source identification”)(Sutherland & Saunders 1992 “On the likelihood ratio for source identification”)

Spectral energy density distribution probability p(i)SED

proportional to the probability that a given source class (i) shows the observed SED

(i) already problematic for the individual blazar

-> 3C279 in flare state

(ii) even more problematic for the blazar population

-> blazar unification scheme is a model

Obviously, there is difference between testing a model and deriving gamma-ray source identifications

(iii) ambiguity between similar SED templates of different source classes

PSR/INS

QSOcand. INS

-> assumptions build on the baseline template SED will aim to discriminate between source classes, but will fail in the individual class already [here only good samples shown, we expect MUCH more sparsely sampled SEDs]

source variability probability p(i)varproportional to the probability that a given source class (i) shows the observered

variability

EGRET experience: variability predominantly used to rule out membership in classes, and only when exhibited at significant level

Obvious: variabilty ID assignment ambiguous if more than 2 classes involved!(low-lat variable sources, high-lat steady sources in EGRET already pending!)quests:

quantify AGN in quiescent state ? non-repeating transients vs. repeated AGN flaring vs. QSO stochastic var.similar variability predictions for different source classes

Independently on how pvar may be determined [0…1], unpredictable quiescent periods for objects believed to exhibit flux variabilty will bring pc = ppos x p(i)

SED x p(i)var x p(i)ext x … -> 0!

This is apparent already for identified EGRET blazars !-> see fractional variability index for individual AGN in Nolan et al. 2003 The sheer number of possible LAT measurements will help here only marginally since this problem is coupled to intrinsic timescales of source activity (where we don’t have not many clues at present!)

source extension probability p(i)ext

proportional to the probability that a given source class (i) shows theobserved extension

This is almost impossible to achieve, since it couples characteristics of an individual source with population properties Since individually identified representatives may constitute the population, the reversal is certainly not generally true!i.e.: Coma cluster of galaxiesi.e.: Coma cluster of galaxies -> extension of diffuse emission deduced from X-ray ~ 2...3°

well beyond LAT psf A2255A2255 -> extension of diffuse emission deduced from X-ray ~ 8..10’

order of LAT psf

i.e.: Cygnus OB2 (i.e.: Cygnus OB2 (TeV J2032+4131) -> extended VHE sourceTeV J2032+4131) -> extended VHE source

…the total number of OB stars alone is expected to be ~2600 (Knödlseder 2003)-> individual counterparts per se inappropriate for understanding this scientific problem, not to mention assigning individual identification probabilities

Potential solution of problem:

An ideal (“lasting”) -ray source catalog will consist only gamma-ray observables(Booooooooooh!)

1st order compromise: catalog will include only rock-solid IDs without extensivecounterpart listings

-> in contrast to EGRET catalogs, there must be a procedure set up and described on what and how is executed for the considered individual source class

i.e. PSR by timing with contemporaneous ephemerides,down to a statistical significance of x in i.e. the Rayleigh/H/…-test

2nd order compromise: catalog will list nearby counterparts without quoting anyother IDs than 1st order compromise

IDs to be derived in respective working groups, but distribution/interaction scheme between science groups tbd.

Individual identification papers and a comprehensive population study shouldaccompany the publication of the 1st year LAT source catalog

Science groups continuously interact with catalog group (receive gamma-ray source observables, feeding catalog with IDs)

-> approach for population study: wait for talk by Diego Torres