title: Excess astrophysical photons from a 0.1-1 keV cosmic axion background.
abstract: Primordial decays of string theory moduli at z~10(12) naturally generate a dark radiation cosmic axion background with 0.1-1 keV energies. This cosmic axion background can be detected through axion-photon conversion in astrophysical magnetic fields to give quasithermal excesses in the extreme ultraviolet and soft x-ray bands. Substantial and observable luminosities may be generated even for axion-photon couplings <<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.
authors.affiliation: Rudolph Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, OX1 3NP Oxford, United Kingdom.
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Conlon
authors.forename: M C David
authors.initials: MC
authors.lastname: Marsh