Particle acceleration in active galactic nuclei

Abstract

Multifrequency observations of Active Galactic Nuclei (AGN) from radio to the X-ray band have now provided evidence that they are site of production of high temperature plasmas (kT > 100 MeV) with strong tails of suprathermal particles. This hot phase is necessarily mixed with a distribution of dense, cold clouds, which scatter, absorb and reprocess the radiation from the hot plasma and relativistic particles, and contribute to form the observed spectra. The generation of high temperature, relativistic plasmas and suprathermal particles is a very challenging problem, not only in the framework of the study of AGN as such, but also in the more general context of the origin of high-energy cosmic rays. In this review we discuss the physical processes which have been suggested to operate in AGN, with special attention for the aspects connected with the generation of specific and/or bulk relativistic energies. Extended radio sources are discussed in these Proceedings by dr. Scheuer as a typical environment where ultra-high-energy cosmic rays (UHECR) may be produced, either by mechanisms based on ordered fields or by stochastic acceleration processes. AGN, which are often associated with them, might appear to be an even better source of high energy particles because of their compactness and global energetics. However they are characterized by high photon densities; correspondingly photomeson losses reduce the final specific energies of the particles allowed to escape. In this sense we may conclude that AGN deliver large amounts of energy into the surrounding components (for instance the extended radio lobes), but are not likely to be the prime generators of the nuclear component of cosmic rays above 10 18 eV. Conversely the neutrino component might be an important product of compact galactic cores and, as such, would represent in perspective a method for investigating AGN. We conclude with some remarks about the possibility of further accelerating energetic particles emerging from AGN in the large scale structures of the local Supercluster, as a viable alternative to explain the generation of extragalactic UHECR.