Active galactic nuclei as high energy engines

Abstract

Active Galactic Nuclei are considered as possible sites of cosmic ray acceleration and some of them have been observed as high energy gamma ray emitters (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 collisions of very high energy protons with soft photons. Moreover, it is often said that electrons cannot reach enough energy through Fermi acceleration to account for the highest energy photons observed with ground Cerenkov telescopes. In this paper, we discussed these points and show that the fast variability of the flares recently observed rules out the assumption of a Fermi acceleration of protons. We show that Fermi acceleration of electrons is enough to account for the gamma spectra, their shape, cutoff and their variability. Moreover, the spectral break is nicely explained by invoking an gamma ray photosphere. Nevertheless, we give estimates of the high energy cosmic 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.