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2.4 The gamma-ray Continuum

Until recently, the only QSO which had been detected in gamma-rays was 3C 273. 76 However, the Compton Gamma Ray Observatory has detected about 14 AGN so far, most of which are highly polarized compact radio sources. 77 For instance, the OVV 3C 279 has shown a luminosity 78 ~ 1048 (Omega / 4pi) erg s-1 between 100 MeV and 10 GeV, where Omega is the solid angle. The apparent superluminal motion in this source should mean that beaming is important. Thus, Omega ~ 1 sr yields a gamma-ray luminosity ~ 9 x 1046 erg s-1, which is still about one order of magnitude higher than the total emission in all other spectral bands. If this also applies to other sources, the dominant radiation from jets may be gamma-rays.

It is unclear at present how this radiation is produced. Pelletier 79 has argued that the jet should be composed of electron-positron pairs, whose outflow rate would be ~ 10-3 Msun yr-1. The Doppler-boost of the luminosity then indicates a Lorentz factor gamma ~ 20. Maraschi et al. 80, 81 suggested that the entire IR to gamma-ray spectrum of 3C 279 may be explained in terms of an extended version of the inhomogeneous relativistic jet model proposed by Ghisellini et al., 82 and found a Lorentz factor in the range 5-14. They also argued that multiwavelength variability studies may yield information about jets closer to the central engine than feasible with VLBI observations.

Sources with high-energy cutoffs in their gamma-ray spectra may be used to constrain pair models. Fabian 33 concluded that the evidence of significant pair production in AGN seems weak at present.