ABSTRACT
One of the major surprises from the EGRET observations on the Compton Gamma Ray Telescope was the large number of extragalactic sources discovered. Although the sky had been surveyed by the earlier SAS-2 and COS-B missions and many of the EGRET sources were of sufficient intensity to have been detected by them, the only extragalactic high energy gamma-ray source known prior to the launch of CGRO was the nearby quasar, 3C273 [10]. COS-B detected 3C273 but the observed spectrum seemed to steepen above 100 MeV and no time variability was observed. Hence, the COS-B team choose to concentrate their observing program on what seemed to be more interesting, the galactic plane. In so doing, they missed out on the very exciting field of extragalactic gamma-ray astronomy. In recent years, HE gamma rays have come to play an important role in the study of AGN and, potentially, in other extragalactic systems.
