Active galactic nuclei as high energy engines

Active Galactic Nuclei are considered as possible sites of cosmic ray accel eration and some of them have been observed as high energy gamma ray emitte rs (blazars). There naturally comes an appealing idea that the acceleration of the highest energy cosmic rays in the AGNs has a signature in the form of gamma ray emission and high energy neutrino emission through the collisi ons of very high energy protons with soft photons. Moreover, it is often sa id that electrons cannot reach enough energy through Fermi acceleration to account for the highest energy photons observed with ground Cerenkov telesc opes. In this paper, we discussed these points and show that the fast varia bility of the flares recently observed rules out the assumption of a Fermi acceleration of protons. We show that Fermi acceleration of electrons is en ough to account for the gamma spectra, their shape, cutoff and their variab ility. Moreover, the spectral break is nicely explained by invoking an gamm a ray photosphere. Nevertheless, we give estimates of the high energy cosmi c ray generation in AGNs and of the resultant neutrino flux, that turns out to be very sensitive to the spectral index of the proton distribution. (C) 1999 Elsevier Science B.V. All rights reserved.