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The extreme characteristics of blazars can be accounted for by the unification hypothesis - that is, by saying that blazars are radio galaxies with jets pointing at us rather than in the plane of the sky. The confirmation of this hypothesis was the discovery with the EGRET experiment on the Compton Gamma-Ray Observatory that blazars are very strong gamma-ray emitters (Fichtel 1994, von Montigny et al. 1995).

The superluminal quasar 3C 279, the first gamma-ray blazar discovered with EGRET, was at the time of its discovery one of the brightest gamma-ray sources in the sky (Hartman et al. 1992). At that epoch, 1991 June, it was undergoing a flare, more than doubling its gamma-ray intensity (at ~ 1 GeV) in a few days (Kniffen et al. 1993). The Parkes radio source 1622-297 has flared even more dramatically, becoming 5-10 times brighter than the high state of 3C 279 in 1991 June, doubling or quadrupling its flux in half a day (Mattox et al. 1997). Other blazars have shown similar rapid flaring at gamma-ray energies; basically, any blazar whose count rate is high enough to detect on short time-scales with EGRET is seen to vary on those time-scales. Doubling times as short as minutes have been seen at TeV energies in one blazar, Mrk 421 (Gaidos et al. 1996).

That so many high energy photons emerge from what appears to be such a compact volume leads to the conclusion that the gamma rays are relativistically beamed. The argument goes like this: the emission region cannot be as compact as it appears (i.e., the gamma-ray photon density cannot be as high) or the gamma-rays would interact with ambient X-ray photons to make pairs, thus preventing the observed gamma-ray emission. With beaming, the rest-frame gamma-ray photon densities are dramatically smaller than implied if the luminosity is emitted isotropically, and the source dimension is also somewhat larger. Thus, that gamma-rays are observed at all implies that blazars are beamed (Dondi & Ghisellini 1995).