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

1. Anticipated problems in our current approach to identify gamma-ray sources or: How far automated source identification might succeed before we will struggle Olaf Reimer & Diego F. Torres (Stanford) (LLNL)

2. status quo: presentations from joint diffuse/catalog workshop earlier this year Counterpart listings / probability

3. Counterpart listings / probability What’s problematic here? Let’s review some examples: An “average” EGRET source: 3EG J1249-8330 [95 =0.66 ° , 2 x 10-7 ph cm-2 s-1] 1) 4 XMM-EPIC pointing -> 148 X-ray sources 2) statistical evaluation of counterparts 3) 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 numerically undistinguishable in the systematics of its computation

4. LS5039 Counterpart listings / probability What’s problematic here? more examples: An EGRET source in the galactic plane 3EG J1824-1514 [~4 x 10-7 ph cm-2 s-1] 1) counterparts in existing catalogs: many already & even more when proceeding into specialized catalogs 2) complicated region in terms of diffuse count prediction: comparable large uncertainty region! 3) 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 by considering two catalogs only) [NRAO VLA Sky Survey (NVSS) + RASS FSC]

5. Counterpart listings / probability What’s problematic here? Argument: LAT will have better source locations, so here a better example from EGRET A bright EGRET source at high galactic latitudes 3EG J1835+5918 [6 x 10-7 ph cm-2 s-1!] 1) counterparts in existing catalogs: Ø 2) dedicated deep HRI pointing: 10 counterparts with almost similar characteristics -> MWL follow-ups 3) 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 dominatespc

6. But source is extended! Counterpart listings What’s problematic here? Argument: LAT will be even better than this, so here an example from VHE gamma-rays HESS J1303-631 (13h03m00.4s±4.4s and δ=−63°11’55”±31”) at least 5 catalog counterparts listed in several counterpart categories 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)

7. After visually appealing cases a more general discussion: positional coincidence probabilityppos (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, which will make it impossible to define a uniform ppos over 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

8. Why did it worked for EGRET ? (“A” blazars / Mattox et al. 97,01 / Soward-Emmered et al. 03-05) Only the dominant population of high-energy gamma-ray emitters has been probed (blazar-class AGN) It does not work at all for galactic sources, i.e. we never proceeded from statistical evidence for SNRs (Dermer & Sturner, Esposito et al., Romero et al. to individual IDs ! Why does it works in X-ray astronomy? ppos <-> small psf in focusing X-ray telescopes resulting in sheer dominance of ppos (+ ability to handle source extension) also: the apparatus is well-defined for arcsec-psf point-sources (Sutherland & Saunders 1992 “On the likelihood ratio for source identification”)

9. PSR/INS cand. INS mQSO 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 -> 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]

10. source variability probability p(i)var proportional 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. mQSO 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!)

11. source extension probability p(i)ext proportional to the probability that a given source class (i) shows the observed 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 galaxies -> extension of diffuse emission deduced from X-ray ~ 2...3° well beyond LAT psf A2255 -> extension of diffuse emission deduced from X-ray ~ 8..10’ order of LAT psf i.e.: Cygnus OB2 (TeV 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

12. Potential solution of problem: An ideal (“lasting”) g-ray source catalog will consist only gamma-ray observables (Booooooooooh!) 1st order compromise: catalog will include only rock-solid IDs without extensive counterpart 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 s in i.e. the Rayleigh/H/…-test 2nd order compromise: catalog will list nearby counterparts without quoting any other 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 should accompany 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