Axionic Dark Radiation and the Milky Way's Magnetic Field

Abstract: Recently it has been suggested that dark radiation in the form of axions produced during the decay of string theory moduli fields could be responsible for the soft x-ray excess in galaxy clusters. These soft X-ray photons come about due to the conversion of these axions into photons in the magnetic fields of the clusters. In this work we calculate the conversion of axionic dark radiation into X-ray photons in the magnetic field of our own Galaxy. We consider Delta N ~ 0.5 worth of dark radiation made up of axions with energy of order 0.1-1 keV. We show that it is possible, if a little optimistic, to explain the large regions of X-ray emission located above and below the centre of the Galactic plane detected in the 3/4 keV ROSAT all sky map completely due to the conversion of dark radiation into photons with an inverse axion-photon coupling of M ~ 3e13 GeV and an axion mass of m < 1e-12 eV. Different parameter values could explain both these features and the 3/4 keV X-ray background. More conservatively, these X-ray observations are a good way to constrain such models of axionic dark radiation.