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An observability study for the tentatively identified 3EG sources likely to be detected by the next-generation Cherenkov telescopes

We present a compilation of data on the 22 tentatively identified gamma-ray sources from the Third EGRET Catalog which may be detected by the next-generation imaging atmospheric Cherenkov telescopes.


INTRODUCTION
The Third EGRET Catalog (3EG, Hartman et al. 1999), comprises 271 objects. Among these, 197 are not identified with a counterpart at lower wavelengths (radio, optical or X-rays). Seven of these are now believed to be artefacts of the background model near bright sources. The remaining 190 are the unidentified EGRET objects (UNIDs). A sizable number of researchers is working on identifying the UNIDs and so far more than 38 have a published tentative ID which still needs to be confirmed by either more observations or improved analysis of archival data. We call these sources the tentatively identified EGRET objects (TIDs).
These new instruments will reach thresholds below 100 GeV and source location accuracies of about 1'. All UNIDs are unidentified because their position is only known with insufficient accuracy, some of the position probability maps having 95% confidence level contour radii of more than 1 • . With an order of magnitude increase in location accuracy, deep well-targeted observations in the radio, optical and X-ray range become possible and make an identification almost certain. In addition, the much improved photon statistics of CTs (collection areas > 10 4 m 2 ) result in a higher sensitivity for pulsed components and thus Pulsar identifications. However, CTs can only contribute for those sources which show emission above several 10 GeV.
In Petry (2001), a catalog was compiled which contains all UNIDs which may possibly be detectable by the next-generation Cherenkov telescopes under moderate assumptions about spectral steepening and taking into account the elevation-dependent sensitivity of the instruments. This catalog contains 78 objects. Among them are 22 TIDs. These objects justify a closer examination since for their tentative counterparts various pieces of information exist which are not available for the other UNIDs: We have an exact source position which can be targeted. We know the source type and have therefore at least vague model predictions for the spectrum beyond the EGRET energy range. We have also model predictions for the variability characteristics of the source.
In this article we present first results of our data compilation and studies concerning the 22 TIDs which may exhibit significant emission beyond 10 GeV and for which the next-generation Cherenkov telescopes may provide the clue to their final identification. Table 1 gives a summary of the data presented at the conference. For each object we examine:

THE DATA PRESENTED HERE
• What is the predicted emission of the TID in the energy regime near the threshold of the next-generation Cherenkov telescopes (CTs)? • Which of the four observatories can observe the object? • Is the emission variable? • What is the angular size of the tentative counterpart? • Are bright stars nearby which may influence the sensitivity of the CTs? • Are there neighbouring EGRET objects which may lead to source confusion? • What chances are there for a detection if the tentative identification turns out to be wrong? Will new pointings be necessary?
Due to the limited space in these proceedings, we refer for more detailed information to our poster which can be found in petry-reimer_poster.eps.gz at http://cossc.gsfc.nasa.gov/meetings/Gamma2001/session17/ and viewed using ghostview, and to Petry & Reimer (2001). We give here, however, the complete list of references.

SIMULATED CAMERA RESPONSES
We have examined the predicted response of the VERITAS photomultiplier camera to the starfield at each of the 22 source positions. The results were obtained from a simulation of the VERITAS optics and wavelength-dependent photomultiplier response and are shown here for the first time. The starfield information (star positions and spectra) was extracted from the SKY2000 master star catalog (Sande et al. 1998) which is reasonably complete up to magnitude 9. If not available from the catalog, the U band magnitude was calculated from the B and V magnitude assuming a main sequence star.

CONCLUSION
The next-generation Cherenkov telescopes (CTs) will be able to make an important contribution to the identification of some of the enigmatic unidentified sources of the third EGRET catalog. EGRET UNIDs for which a tentative identification exists are especially easy to target, and instruments on the northern hemisphere will be able to observe almost all such sources for which emission beyond 30 GeV can be expected. The short catalog of 22 such sources which we have compiled here, shows that a positive detection of any of these objects by CTs will be an interesting result in itself providing constraints for source models and, of course, leading to a clear identification of the 3EG source. Furthermore, the lessons learned form the observations of these objects will help in the examination of the remaining 57 EGRET UNIDs from the list compiled in Petry (2001) which have no identification whatsoever but which may have significant emission beyond 30 GeV. For the beginning of an observation campaign, the most interesting object in our list is 3EG J1856+0114. This object has a flat spectrum with no obvious cut-off below 10 GeV. Due to its proximity to the SNR W44, it has been studied extensively (see e.g. the overview in Buckley et al. 1998). W44 is a radio shell-type SNR with an angular diameter of about 0.5 • associated with PSR 1853+01. There is both evidence for a synchrotron nebula and interactions with molecular clouds. Extrapolating the thin outer gap model of  to 60 GeV yields a differential flux of 65 % of the Crab Nebula. The object is an ideal candidate for CT observations and is accidentally the only UNID which can be equally well observed both from the southern and the northern hemisphere. It could therefore be used to cross-calibrate the four CT observatories. 3EG name tent. ID (1) RA/DEC J2000 (2) l/b (3) z (4) α (5) F(60 GeV) (6) observ. (7) var. (8) s. (9) p. (