Galaxy Cluster Thermal X-Ray Spectra Constrain Axion-Like Particles

Abstract

Axion-like particles (ALPs) and photons inter-convert in the presence of a magnetic field. At keV energies in the environment of galaxy clusters, the conversion probability can become unsuppressed for light ALPs. Conversion of thermal X-ray photons into ALPs can introduce a step-like feature into the cluster thermal bremsstrahlung spectrum, and we argue that existing X-ray data on galaxy clusters should be sufficient to extend bounds on ALPs in the low-mass region $m_a\lesssim 1\times 10^{-12}\mathrm{eV}$ down to $M\sim 7\times 10^{11}\mathrm{GeV}$, and that for $10^{11}\mathrm{GeV}<M\lesssim 10^{12}$ GeV light ALPs give rise to interesting and unique observational signatures that may be probed by existing and upcoming X-ray (and potentially X-ray polarisation) observations of galaxy clusters.

Comments: 5 pages