Anisotropies in the diffuse gamma-ray background measured by Fermi LAT

TL;DR

This paper analyzes small-scale anisotropies in the diffuse gamma-ray background measured by Fermi LAT, revealing evidence of unresolved source populations and providing insights into their nature through angular power spectrum analysis.

Contribution

It presents the first detailed measurement of gamma-ray background anisotropies using Fermi LAT data and Monte Carlo simulations to identify contributing source populations.

Findings

Detection of anisotropy above photon noise at <2° scales

Anisotropy energy dependence suggests unclustered sources

Photon index of source populations estimated at 2.40±0.07

Abstract

The small angular scale fluctuations of the (on large scale) isotropic gamma-ray background (IGRB) carry information about the presence of unresolved source classes. A guaranteed contribution to the IGRB is expected from the unresolved gamma-ray AGN while other extragalactic sources, Galactic gamma-ray source populations and dark matter Galactic and extragalactic structures (and sub-structures) are candidate contributors. The IGRB was measured with unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi gamma-ray observatory, and these data were used for measuring the IGRB angular power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky observations were performed to provide a reference against which to compare the results obtained for the real data set. The Monte Carlo simulations are also a method for performing those detailed studies of the APS contributions of single source populations, which are required in order to identify the actual IGRB contributors. We present preliminary results of an anisotropy search in the IGRB. At angular scales <2deg (e.g. above multipole 155), angular power above the photon noise level is detected, at energies between 1 and 10 GeV in each energy bin, with statistical significance between 7.2 and 4.1 sigmas. The energy not dependence of the fluctuation anisotropy is pointing to the presence of one or more unclustered source populations, while the energy dependence of the intensity anisotropy is consistent with source populations having average photon index 2.40\pm0.07.