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

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.

4 pages, 2 figures, revised version, accepted by ApJ Letters

References in corpus (3)

• The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era
• INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth
• Discovery of an Extreme MeV Blazar with the Swift Burst Alert Telescope

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• BASS XXXIII: Swift-BAT blazars and their jets through cosmic time
• Gamma-ray and Neutrino Signals from Accretion Disk Coronae of Active Galactic Nuclei
• Cosmological Evolution of Flat-Spectrum Radio Quasars based on the {\it Swift}/BAT 105-month Catalog and Their Contribution to the Cosmic MeV Gamma-ray Background Radiation
• Cosmic constraints rule out s-wave annihilation of light dark matter
• Dark matter annihilation at cosmological redshifts: possible relic signal from annihilation of weakly interacting massive particles