Absorption of gamma rays in active galaxies as a test of the jet hypothesis

Powerful X-ray emission is a common feature of active galaxies' some of which have also been detected as gamma-ray sources(1-5). The observed high luminosities, coupled with reports of variability on time scales of days or less at X-ray energies(6) and of 1 yr or less at gamma-ray energies(3), suggest that both emissions may originate in the same volume deep within the nucleus of the galaxy. Under this assumption we investigate here the attenuation of gamma rays as a result of pair-production in photon-photon collisions with X rays and show that whilst Seyfert galaxies are transparent to gamma radiation up to similar to 1 GeV, QSOs and BL Lac objects are generally opaque about I MeV. However, the detection of gamma rays from these latter classes of objects may be explained if beaming of the X-gamma emission is assumed.