The Origin of the Extragalactic Gamma-Ray Background and Implications for Dark-Matter Annihilation

TL;DR

This paper models the contribution of blazars to the extragalactic gamma-ray background, showing they account for about half of it and constraining dark matter annihilation based on these findings.

Contribution

It provides an updated luminosity function and spectral model for blazars, quantifies their contribution to the EGB, and constrains dark matter annihilation cross sections.

Findings

Blazars contribute approximately 50% of the EGB above 0.1 GeV.

Fermi-LAT has resolved about 70% of the blazar contribution.

Blazars, star-forming, and radio galaxies explain the EGB spectrum from 0.1 to 820 GeV.

Abstract

The origin of the extragalactic $\gamma$-ray background (EGB) has been debated for some time. { The EGB comprises the $\gamma$-ray emission from resolved and unresolved extragalactic sources, such as blazars, star-forming galaxies and radio galaxies, as well as radiation from truly diffuse processes.} This letter focuses on the blazar source class, the most numerous detected population, and presents an updated luminosity function and spectral energy distribution model consistent with the blazar observations performed by the {\it Fermi} Large Area Telescope (LAT). We show that blazars account for 50$^{+12}_{-11}$\,\% of the EGB photons ($>$0.1\,GeV), and that {\it Fermi}-LAT has already resolved $\sim$70\,\% of this contribution. Blazars, and in particular low-luminosity hard-spectrum nearby sources like BL Lacs, are responsible for most of the EGB emission above 100\,GeV. We find that the extragalactic background light, which attenuates blazars' high-energy emission, is responsible for the high-energy cut-off observed in the EGB spectrum. Finally, we show that blazars, star-forming galaxies and radio galaxies can naturally account for the amplitude and spectral shape of the background in the 0.1--820\,GeV range, leaving only modest room for other contributions. This allows us to set competitive constraints on the dark-matter annihilation cross section.