Imaging of extragalactic low-energy gamma-ray sources: Prospects, techniques, and instrumentation

The handful of extragalactic low-energy gamma-ray sources so far observed are all active galaxies, which are expected to dominate future detections. Improved understanding of the nature of active galaxies will require detailed observations of 10-20 sources, while understanding of their gamma-ray luminosity function and its evolution will require the detection of ∼ 100 sources. The author presents investigations of the sensitivity and angular resolution required of gamma-ray instruments capable of these observations. His estimates of the hard x-ray and low-energy gamma-ray N(>S) relation for active galaxies show that instruments capable of 100 detections will need to operate at signal-to-noise ratios of 10-3-10-4, requiring careful control of systematic errors. He found that the strictest angular-resolution requirements arise not from the need to resolve detectable sources, but from the need to control the level of direction-to direction fluctuations in the diffuse background level. He concludes that gamma-ray instruments capable of detecting 100 active galaxies must have sub-degree angular resolution