Hints of a Cosmic Axion Background

In models of particle physics and cosmology arising from compactifications of string theory , the reheating of the standard model after inflation generically proceeds through the decay of heavy moduli. Non-vanishing branching ratios for moduli decay into axions generate axionic dark radiation which linger in the present universe as a homogeneous and isotropic background of relativistic axions, hence called the Cosmic Axion Background (CAB). Axion-photon conversion in magnetic fields may render the CAB visible, and we show that the soft X-ray excess observed in galaxy clusters may provide hints for the existence of a CAB