On re-acceleration, pairs and the high-energy spectrum of AGN and Galactic black hole candidates

Abstract

OSSE on CGRO and SIGMA on GRANAT have recently shown that the hard X-ray spectra of several Seyfert AGN are steep like the spectra from Galactic black hole candidates. This suggests that the X-rays in these sources are produced by a thermal plasma. There are, however, problems in maintaining a thermal plasma in such rapidly varying sources, since it may not have time to thermalize, especially if the thermalization process is driven by two-body particle interactions. We show here that non-thermal and thermal plasmas produce the same high-energy spectrum if multiple Compton scattering is the main emission mechanism and the maximum energy of the emitting particle population is limited to a few MeV. This will occur in an emission region that is subject to repeated heating, whatever the initial particle spectrum. We propose re-acceleration of electron-positron pairs as a feedback mechanism which can keep the maximum energy of the emitting particles in the required range. Using self-consistent particle and photon distributions, we show that this model can account for the observed spectra.