High-energy radiation from low-luminosity accreting black holes

Abstract

This PhD thesis includes theoretical and observational studies of γ-ray emission from radio-quiet accreting black holes. The theoretical motivation for the search of γ-ray emission from such sources concerns the considerable hadronic production of γ-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 ∼ 10−5LEdd for the X-ray luminosities between ∼ 10−4LEdd and 10−3LEdd. These levels of γ-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 γ-ray signal in the LAT data from NGC 4151, which is only slightly below the formal detection threshold of 5σ. I also found hints for the correlation between the X-ray and γ-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 ∼ 5σ difference between spectral parameters fitted to these datasets, and a similar significance of the reversal of the γ-ray signal in significance maps for low and high γ-ray energies. This X/γ-ray correlation indicates that the γ-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 γ-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the γ-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.