Abstract

Gamma ray emission from extragalactic sources is interpreted as the Doppler boosted annihilation and Inverse Compton radiation from a relativistic electron-positron beam in the frame of the two-flow model. In the case of 3C279, the high luminosity and the rapid variability of gamma ray emission suggest a relativistically moving source, but even so the compactness cannot be smaller than unity at light week scale with a reasonable Doppler factor. This supports the two-flow model of extragalactic radio sources, where the small scale emission comes from a relativistic electron-positron beam, heated by a MHD jet responsible for the large-scale (kpc) radio structures.

Introduction

The GRO satellite has detected intense gamma ray emission from several Active Galactic Nuclei and quasars. Remarkably, all of them are associated with a flat spectrum radio source, whose radio spectral index αr is smaller than 0.5, and half of them exhibit known or probable superluminal motions (the others have not been observed at different epochs in VLBI). Just like the commonly invoked Doppler beaming amplification of radio emission, the high γ-ray luminosity suggests also that the emitting source is moving relativistically.

For 3C279 in particular, the spectrum reported by Hermsen & al. show a maximum emission per logarithmic energy interval around 10 MeV, with a photon spectral index of approximately 1.5 below the turn-over frequency and approximately 2 above it. A rapid flare has been observed with an increase of the luminosity by a factor 5 on a time scale of 2 days.