Since its launch, CGRO has detected more than 20 γ-ray emitting AGN, most of them associated with powerful, radio-loud, flat-spectrum objects, exhibiting VLBI superluminal motions. In the case of 3C279, the huge value of the apparent luminosity (∼ 1048erg.s−1) and the variability time-scale of a few days (Hartmann et al., 1992) gives a very large compacity lapp ≃ 200, that is, the medium should be completely thick to γ-rays. This contradiction can be explained if the γ-rays originate from a relativistic jet pointing at a small angle with respect to the line of sight (Maraschi et al., 1992). However, the still large value of compacity suggests the existence of an inner, more compact region where pair production can take place efficiently (Henri et al., 1993). This supports the so-called “two-flow” model, where the superluminal motion is attributed to the expansion of a relativistic pair plasma heated by a MHD jet from an accretion disk (Sol et al., 1989). Hence we propose to interpret the spectral break observed in many objects around a few MeV (Lichti et al., 1993) by an opacity effect due to photon-photon absorption by pair production.
Change of a Weibel-type to an Alfvénic shock in pair plasma by upstream waves