The Isotropic Radio Background and Annihilating Dark Matter

TL;DR

This paper explores whether annihilating dark matter particles could explain the excess isotropic radio background observed by ARCADE-2, suggesting a specific mass range and annihilation channels consistent with other cosmic observations.

Contribution

It proposes a dark matter annihilation model that accounts for the radio excess and aligns with gamma-ray background measurements, highlighting specific particle properties.

Findings

Dark matter annihilation can explain the radio excess with certain mass and channel constraints.

The required annihilation cross section is comparable to thermal relic predictions.

Dark matter annihilation likely contributes significantly to the gamma-ray background.

Abstract

Observations by ARCADE-2 and other telescopes sensitive to low frequency radiation have revealed the presence of an isotropic radio background with a hard spectral index. The intensity of this observed background is found to exceed the flux predicted from astrophysical sources by a factor of approximately 5-6. In this article, we consider the possibility that annihilating dark matter particles provide the primary contribution to the observed isotropic radio background through the emission of synchrotron radiation from electron and positron annihilation products. For reasonable estimates of the magnetic fields present in clusters and galaxies, we find that dark matter could potentially account for the observed radio excess, but only if it annihilates mostly to electrons and/or muons, and only if it possesses a mass in the range of approximately 5-50 GeV. For such models, the annihilation cross section required to normalize the synchrotron signal to the observed excess is sigma v ~ (0.4-30) x 10^-26 cm^3/s, similar to the value predicted for a simple thermal relic (sigma v ~ 3 x 10^-26 cm^3/s). We find that in any scenario in which dark matter annihilations are responsible for the observed excess radio emission, a significant fraction of the isotropic gamma ray background observed by Fermi must result from dark matter as well.