High-energy gamma-ray emission from active galaxies: Egret observations and their implications

The energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory has so far detected, with a high degree of certainty, 33 active galactic nuclei (AGNs) in high-energy gamma rays (E > 100 MeV) during Phase I (91/16/05-92/17/11) and Phase II (92/17/11-93/07/09). There are detections of 11 other active galaxies with lesser significance. Of the sources detected, the majority are quasars and six are usually classified as BL Lacertae (BL Lac) objects. They seem to be all members of the blazar class a rather well defined subclass of AGN. To aid attempts at relating the observed properties of the EGRET sources to physical models of AGNs, a summary of EGRET AGN observations is presented. This summary includes gamma-ray spectra and time-dependent gamma-ray fluxes of detected sources. For sources where the information is available in the literature, multifrequency spectra covering the range from radio to gamma rays are also included. Constraints are discussed which the EGRET observations place upon various emission models. Finally, the contribution of AGNs to the diffuse extragalactic gamma-ray emission is reexamined in light of the EGRET observations.