GAMMA-RAY EMISSION OF ACTIVE GALACTIC NUCLEI AS A SIGNATURE OF RELATIVISTIC ELECTRON-POSITRON BEAMS

Gamma-ray emission from extragalactic sources is interpreted as the Doppler-boosted annihilation and inverse Compton radiation from a relativistic electron-positron beam. We argue that Doppler boosting is required to avoid a strong depletion by gamma-gamma pair production, but even so, the compactness cannot be smaller than unity at light-week scale with a reasonable Doppler factor. This strongly suggests the existence of a beamed electron-positron source. The case of the quasar 3C 279 is investigated in detail. We show that one can easily account for the observed luminosity and variability time scale with reasonable values of the plasma density and bulk Lorentz factor of the beam, in accordance with previous estimates based upon observations at other wavelengths (radio and X-rays). The observations put new constraints on the size, the geometry, and the particle distribution function of the source. We discuss briefly the link between gamma-ray and radio VLBI observations.