High energy gamma-ray emission from active galactic nuclei observed by the energetic gamma-ray experiment telescope (EGRET)

Abstract

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory has detected 16 active galactic nuclei in high energy gamma rays thus far during the first year in orbit. Of these, the majority are quasars and four are usually classified as BL Lacertae objects. No Seyferts have been seen. Of the AGN’s seen, 3C 279 had the highest flux when first observed, but it is variable and was at a significantly lower level when observed later. There is a wide range of distances among the observed set, with four quasars having known z values in excess of 1.0. One BL Lac has a z of 0.94. If the photon spectra are represented by power laws in energy, those which have been determined have spectral indices ranging from about −1.7 to −2.4. In some cases, there is a suggestion of a steepening at the highest energies observed.