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

TL;DR

This paper proposes a new spectral model for AGN coronae including nonthermal electrons, successfully explaining the MeV gamma-ray background and suggesting magnetic reconnections as the corona heating mechanism.

Contribution

The study introduces a nonthermal electron component into AGN spectral models, providing a better fit to the MeV gamma-ray background and supporting magnetic reconnection as the corona heating process.

Findings

The MeV background spectrum is explained by the nonthermal electron component.

The nonthermal electron spectral index is similar to that in solar flares.

Future MeV detectors could observe the predicted nonthermal flux from nearby AGNs.

Abstract

The origin of the extragalactic gamma-ray background radiation at 1-10 MeV is still unknown. Although the cosmic X-ray background (CXB) up to a few hundreds keV can be accounted for by the sum of Active Galactic Nuclei (AGNs), current models of AGN spectra cannot explain the background spectrum beyond ~1 MeV, because of the thermal exponential cutoff of 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 hot corona if it is heated by magnetic reconnections. We show that the MeV background spectrum can nicely be explained 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 d(ln N_e)/d(ln E_e) ~ -4. Although the MeV gamma-ray flux from such a component in nearby AGN spectra is below the detection limit of past observations, it could be detected by planned future MeV detectors. We point out that the amount of the nonthermal component and its electron index are similar to those found for electrons accelerated by magnetic reconnections in solar flares and the Earth magnetosphere, giving a support to the reconnection hypothesis for the origin of hot AGN coronae.