Comment on "Joint Anisotropy and Source Count Constraints on the Contribution of Blazars to the Diffuse Gamma-Ray Background"

TL;DR

This paper critiques a previous study on blazars' contribution to the gamma-ray background, showing that more realistic models suggest blazars account for a larger fraction of the background than previously claimed.

Contribution

It demonstrates that previous assumptions oversimplify blazar populations, and provides a more accurate estimate of their contribution to the gamma-ray background based on physical models.

Findings

Physical models can explain up to 60% of the gamma-ray background.

Simplified assumptions underestimate blazar contribution to the background.

Spectral information significantly impacts anisotropy calculations.

Abstract

We show the conclusions claimed in the manuscript arXiv:1202.5309v1 by Cuoco, Komatsu and Siegal-Gaskins (CKS) are not generally valid. The results in CKS are based on a number of simplifying assumptions regarding the source population below the detection threshold and the threshold flux itself, and do not apply to many physical models of the blazar population. Physical blazar population models that match the measured source counts above the observational threshold can account for 60% of the diffuse gamma-ray background intensity between 1-10 GeV, while the assumptions in CKS limit the intensity to <30%. The shortcomings of the model considered in CKS arise from an over-simplified blazar source model. A number of the simplifying assumptions are unjustified, including: first, the adoption of an assumed power-law source-count distribution, dN/dS, to arbitrary low source fluxes, which is not exhibited in physical models of the blazar population; and, second, the lack of blazar spectral information in calculating the anisotropy of unresolved gamma-ray blazar emission. We also show that the calculation of the unresolved blazars' anisotropy is very sensitive to the spectral distribution of the unresolved blazars through the adopted source resolution threshold value, and must be taken into account in an accurate anisotropy calculation.