Gamma-Ray Blazars

Roughly 60 blazars have been identiied with > 100 MeV EGRET gamma-ray sources, and exhibit inferred isotropic luminosities as large as 3 10 49 ergs s ?1. The two most remarkable characteristics about the EGRET observations are that the-ray luminosity often dominates the bolometric power, and that variability on timescales at least as short as a day is seen from several sources in their aring states. Gamma-ray blazars have been detected with red-shifts ranging from z = 0:03 to z = 2:3, and the distribution of redshifts is similar to that of the at spectrum radio quasars. The distribution of > 100 MeV pho-ton spectral indices for strong detections is well described by a Gaussian function peaking near 2.1 with a standard deviation of 0.4. Blazar spectra at hard X-ray and soft-ray energies have harder spectra than in the EGRET range and, from analyses of contemporaneous data sets, the change in spectral index clearly exceeds 0.5 for 3C 273 and marginally exceeds 0.5 for PKS 0528+134. An index change of 0.5 is predicted by incomplete Compton cooling models. COMPTEL observations reveal a separate class of 2 identiied blazars and 3 unidentiied candidates peaking at MeV energies with narrow widths. In some cases, are spectra at > 100 MeV energies are harder than spectra in quiescence. The combined luminosity and variability data strengthen arguments for rel-ativistic beaming or bulk outtow from the active nucleus. Gamma-ray transparency arguments are based on the pair-production attenuation of gamma rays in a stationay region deened by the variability time scale, and many violations are indicated. The Elliot-Shapiro relation compares the minimum black hole mass implied by the Eddington limit with the maximum black hole mass implied by the variability time scale related to the black hole Schwarzschild radius; weak violations have been observed for PKS 0528+134 and PKS 1622-297. Most models adopt a beaming paradigm, with one class of models considering leptons as the directly accelerated and radiating particles, and the other class considering hadrons. In the former class, Compton scattering is favored to make the gamma-ray emission, though opinions diier on whether the soft photons are primarily internal synchrotron photons or external photons from the accretion disk which enter the jet either directly or after scattering oo clouds. Hadronic models involve secondary production or photomeson production followed by pair cascades, and predict associated neutrino production.