Flat spectrum radio quasars and BL Lacs dominate the anisotropy of the unresolved gamma-ray background

TL;DR

This study dissects the gamma-ray sky's anisotropy, revealing that flat spectrum radio quasars and BL Lacs dominate the unresolved gamma-ray background, with implications for dark matter searches.

Contribution

It identifies the key source populations responsible for gamma-ray background anisotropy and quantifies their contributions using luminosity functions and spectral assumptions.

Findings

FSRQs dominate at low energies

BL Lacs dominate at higher energies

Blazars contribute up to 30% of gamma-ray background

Abstract

We analyze the angular power spectrum (APS) of the unresolved gamma-ray background (UGRB) emission and combine it with the measured properties of the resolved gamma-ray sources of the Fermi-LAT 4FGL catalog. Our goals are to dissect the composition of the gamma-ray sky and to establish the relevance of different classes of source populations of active galactic nuclei in determining the observed size of the UGRB anisotropy, especially at low energies. We find that, under physical assumptions for the spectral energy dispersion, i.e. by using the 4FGL catalog data as a prior, two populations are required to fit APS data, namely flat spectrum radio quasars (FSRQs) at low energies and BL Lacs (BLLs) at higher energies. The inferred luminosity functions agree well with the extrapolation of the FSRQ and BLL ones obtained from the 4FLG catalog. We use these luminosity functions to calculate the UGRB intensity from blazars, finding a contribution of 20% at 1GeV and 30% above 10 GeV. Finally, bounds on an additional gamma-ray emission due to annihilating dark matter are derived.