Searching for a 0.1-1 keV Cosmic Axion Background

TL;DR

This paper explores the potential detection of a cosmic axion background generated by primordial string moduli decays, which could explain soft X-ray excesses in galaxy clusters and contribute to the cosmic X-ray background.

Contribution

It proposes a novel detection method via axion-photon conversion and links primordial string theory processes to observable X-ray phenomena.

Findings

Axion-photon conversion can produce observable X-ray excesses.

The cosmic axion background may explain soft X-ray emissions from galaxy clusters.

Potential contribution to the diffuse cosmic X-ray background.

Abstract

Primordial decays of string theory moduli at z \sim 10^{12} naturally generate a dark radiation Cosmic Axion Background (CAB) with 0.1 - 1 keV energies. This CAB can be detected through axion-photon conversion in astrophysical magnetic fields to give quasi-thermal excesses in the extreme ultraviolet and soft X-ray bands. Substantial and observable luminosities may be generated even for axion-photon couplings \ll 10^{-11} GeV^{-1}. We propose that axion-photon conversion may explain the observed excess emission of soft X-rays from galaxy clusters, and may also contribute to the diffuse unresolved cosmic X-ray background. We list a number of correlated predictions of the scenario.