SUPERLUMINAL BLAZARS AND THE EXTRAGALACTIC GAMMA-RAY BACKGROUND

The detection of a few dozen extragalactic gamma ray blazars of extrem ely high luminosity by the EGRET instrument on board the Compton Gamma Ray Observatory appears to suggest that blazars make the overwhelming contribution to the cosmic gamma ray background in the energy range 1 00 MeV-10 GeV. In this paper we point out that the superluminal effect which boosts and beams the gamma ray emission in the jets of blazars will flatten the source count in the low flux part. Consequently, the unresolved blazars would not be expected to make much contribution to the gamma ray background. From our direct modelling of the source coun t, we conclude that the contribution of the unresolved blazars to the gamma ray background is only 10% of the latest estimate of the EGRET d ata. The implication for the cosmological evolution of the blazars is discussed.