The Cosmic MeV Gamma-ray Background and Hard X-ray Spectra of Active Galactic Nuclei: Implications for the Origin of Hot AGN Coronae

摘要

The origin of the extragalactic gamma-ray background radiation at 1-10 MeV isstill unknown. Although the cosmic X-ray background (CXB) up to a few hundredskeV can be accounted for by the sum of Active Galactic Nuclei (AGNs), currentmodels of AGN spectra cannot explain the background spectrum beyond ~1 MeV,because of the thermal exponential cutoff of electron energy distributionassumed in the models. Here we construct a new spectral model by calculatingthe Comptonization process including nonthermal electrons, which are expectedto exist in an AGN hot corona if it is heated by magnetic reconnections. Weshow that the MeV background spectrum can nicely be explained by our model,when coronal electrons have a nonthermal power-law component whose total energyis a few percent of the thermal component and whose spectral index is d(lnN_e)/d(ln E_e) ~ -4. Although the MeV gamma-ray flux from such a component innearby AGN spectra is below the detection limit of past observations, it couldbe detected by planned future MeV detectors. We point out that the amount ofthe nonthermal component and its electron index are similar to those found forelectrons accelerated by magnetic reconnections in solar flares and the Earthmagnetosphere, giving a support to the reconnection hypothesis for the originof hot AGN coronae.