Lower Limits on the Anisotropy of the Extragalactic Gamma-Ray Background implied by the 2FGL and 1FHL Catalogs

TL;DR

This paper investigates the limits of anisotropy in the extragalactic gamma-ray background using Fermi-LAT catalogs, revealing larger anisotropies at high energies than previously thought, which impacts models of unresolved sources.

Contribution

It provides an independent consistency check of EGRB anisotropy limits by comparing different Fermi-LAT catalogs, challenging previous assumptions about source contributions.

Findings

Evidence for larger anisotropies above 5 GeV than previously reported

Uncertainty in anisotropy limits affects constraints on high-redshift gamma-ray sources

Smooth extensions of known sources could explain both isotropic and anisotropic EGRB components

Abstract

In principle, the angular anisotropy in the extragalactic gamma-ray background (EGRB) places severe constraints upon putative populations of unresolved gamma-ray point sources. Existing estimates of the EGRB anisotropy have been constructed by excising known point sources, e.g., taken from the First or 2 Year Fermi-LAT Source Catalog (1FGL or 2FGL, respectively) and statistically analyzing the residual gamma-ray sky maps. We perform an independent check of the EGRB anisotropy limits by comparing the values obtained from the 1FGL-masked sky maps to the signal implied by sources that lie below the 1FGL detection threshold in the more sensitive 2FGL and 1FHL (First Fermi-LAT catalog of >10 GeV sources). As such, our analysis provides an internal consistency check of implications for source counts and spectral index distributions of gamma-ray bright active galactic nuclei obtained from Fermi-LAT data. Based on this, we find evidence for substantially larger anisotropies than those previously reported at energies above 5 GeV, where BL Lac objects are likely to provide the bulk of their contribution to the EGRB. This uncertainty in the EGRB anisotropy cautions against using it as an independent constraint for the high-redshift gamma-ray universe. Moreover, this would suggest that contrary to previous claims, smooth extensions of the resolved point-source population may be able to simultaneously explain both the isotropic and anisotropic components of the EGRB.