The detectability of dark matter annihilation with Fermi using the anisotropy energy spectrum of the gamma-ray background

TL;DR

This paper explores how the anisotropy energy spectrum of gamma-ray background measured by Fermi can reveal dark matter annihilation signals, especially in the presence of unresolved blazars, by detecting characteristic modulations.

Contribution

It demonstrates the potential to detect dark matter annihilation signatures through anisotropy spectrum analysis, considering realistic astrophysical backgrounds and proposing improved likelihood methods.

Findings

Detectable dark matter signals are possible with current Fermi data.

Pair-annihilation cross sections near thermal relic values can produce observable features.

The anisotropy energy spectrum provides a new avenue for dark matter detection.

Abstract

The energy-dependence of the anisotropy (the anisotropy energy spectrum) of the large-scale diffuse gamma-ray background can reveal the presence of multiple source populations. Annihilating dark matter in the substructure of the Milky Way halo could give rise to a modulation in the anisotropy energy spectrum of the diffuse gamma-ray emission measured by Fermi, enabling the detection of a dark matter signal. We determine the detectability of a dark-matter-induced modulation for scenarios in which unresolved blazars are the primary contributor to the measured emission above ~1 GeV and find that in some scenarios pair-annihilation cross sections of order the value expected for thermal relic dark matter can produce a detectable feature. We anticipate that the sensitivity of this technique to specific dark matter models could be improved by tailored likelihood analysis methods.