High-Energy Gamma Rays from the BL Lacertae Object PKS 2155−304

We present the first detection of gamma-ray emission from the extragalactic source PKS 2155-304, which is often considered to be a prototype of X-ray-selected BL Lacertae objects. The 30 MeV-10 GeV photon spectrum derived from measurements taken with EGRET on the Compton Observatory during 1994 November 15-29 indicates a very hard spectrum with a power-law spectral index of 1.71 ± 0.24. This spectral hardness when combined with the proximity of the source ( z = 0.116) makes it an excellent candidate for detection at TeV gamma-ray energies. We briefly discuss the implications of these gamma-ray measurements for models of BL Lac objects.