Extragalactic Very-High-Energy gamma-ray background

TL;DR

This study analyzes Fermi telescope data to understand the origin of the extragalactic gamma-ray background, revealing that unresolved BL Lac objects could account for the observed spectrum above 30 GeV.

Contribution

It demonstrates that the extragalactic gamma-ray background at high energies can be explained by unresolved BL Lac objects with specific cosmological evolution.

Findings

EGB spectrum above 30 GeV matches BL Lac spectra

BL Lac population shows positive cosmological evolution with 1<k<4

Unresolved BL Lacs can fully explain EGB at E>30 GeV

Abstract

We study the origin of the extragalactic diffuse gamma-ray background using the data from the Fermi telescope. To estimate the background level, we count photons at high Galactic latitudes |b|>60 degrees. Subtracting photons associated to known sources and the residual cosmic ray and Galactic diffuse backgrounds, we estimate the Extragalactic Gamma-ray Background (EGB) flux. We find that the spectrum of EGB in the very-high-energy (VHE) band above 30 GeV follows the stacked spectrum of BL Lacs. LAT data reveal the positive (1+z)^k, 1<k<4 cosmological evolution of the BL Lac source population consistent with that of their parent population, FR I radio galaxies. We show that EGB at E>30 GeV could be completely explained by emission from unresolved BL Lacs if k~3.