The cosmic MeV gamma-ray background and hard X-ray spectra of active galactic nuclei: Implications for the origin of hot AGN coronae

The origin of the extragalactic gamma-ray background radiation at 1-10 MeV is still unknown. Although the cosmic X-ray background up to a few hundred keV can be accounted for by the sum of active galactic nuclei (AGNs), current models of AGN spectra cannot explain the background spectrum beyond similar to 1 MeV, because of the thermal exponential cutoff of the electron energy distribution assumed in the models. Here we construct a new spectral model by calculating the Comptonization process, including nonthermal electrons, which are expected to exist in an AGN's hot corona if it is heated by magnetic reconnections. We show that the MeV background spectrum can be explained nicely by our model, when coronal electrons have a nonthermal power-law component whose total energy is a few percent of the thermal component and whose spectral index is. Although d ln N-e/d ln E-e approximate to -4 in nearby AGN spectra the MeV gamma-ray flux from such a component is below the detection limit of past observations, it might be detected by MeV detectors that are planned for future use. We point out that the nonthermal component's total energy and electron index are similar to those found for electrons accelerated by magnetic reconnections in solar flares and in the Earth's magnetosphere, which supports the reconnection hypothesis, which in turn explains the origin of an AGN's hot coronae.