High Energy Radiation From Low-luminosity Accreting Black Holes

This PhD thesis includes theoretical and observational studies of gamma-ray emission from radio-quiet accreting black holes. The theoretical motivation for the search of gamma-ray emission from such sources concerns the considerable hadronic production of gamma-rays predicted by models of hot flows, which most likely power these sources at low luminosities. I thoroughly investigated this model prediction and I found that the luminosity at either hundreds of MeV or in the GeV range, depending on proton distribution, can reach ~10-5 LEdd for the X-ray luminosities between ~10-4LEdd and 10-3LEdd. These levels of gamma-ray luminosities can be probed in some Seyfert galaxies. Comparing the model predictions with Fermi/LAT upper limits for NGC 4258, NGC 7213 and NGC 4151 I found interesting constraints on the acceleration efficiency of protons, plasma magnetization and black hole spins. I found an interesting hint for a gamma-ray signal in the LAT data from NGC 4151, which is only slightly below the formal detection threshold of 5 sigma. I also found hints for the correlation between the X-ray and gamma-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. I have divided the Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low and high level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. I found a 5 sigma difference between spectral parameters fitted to these datasets, and a similar significance of the reversal of the gamma-ray signal in significance maps for low and high gamma-ray energies. This X/gamma-ray correlation indicates that the gamma-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. I compared NGC 4945 with other starburst galaxies detected by LAT and I note similarities between those with active nuclei, e.g. unlikely high efficiencies of gamma-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the gamma-ray emission. Finally, I took into account AGILE observations of Cyg X-1. I found that it has not reached yet a sensitivity level needed for a detection of this source.