Annihilation of Positrons from AGN Jets As a Possible Source of Cosmic Gamma-Ray Background at Energies below 511 keV

Abstract

The origin of the diffuse gamma-ray background in the range from hundreds keV to several MeV is not known conclusively. From current models and observations it is believed that, at least partially, this background is formed by blazars and remnants of supernovae (SN) of type Ia in distant galaxies. However, these contributions are not sufficient to reproduce the observed level of the signal. In this work we propose another source which could contribute to this background, namely the jets of active galactic nuclei (AGN). The composition of jets is not known, but there are observational hints that the fraction of positrons there is substantial. Positrons are partially evacuated to the intergalactic medium and partially mix with the circumgalactic medium and annihilate there comparatively quickly. Using the AGN luminosity function, we estimated the positron production rate and the contribution of the positron annihilation to the cosmic background below 511 keV. We also estimated the analogous contribution from positron annihilation within SN Ia remnants in distant galaxies. The contribution of AGNs is estimated to be a factor of 5–10 smaller than the observed background intensity, and the contribution from SNe is yet smaller by one order of magnitude. Nevertheless, the contribution of AGNs appeared to be larger than the contribution of blazars estimated from Swift-BAT and Fermi-LAT observations. The main uncertainty in our model is the fraction of positrons remaining in the circumgalactic medium which makes our estimation an upper limit.