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 ma≲1×10−12eV down to M∼7×1011GeV, and that for 1011GeV<M≲1012 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.
@article{arxiv.1509.06748,
title = {Galaxy Cluster Thermal X-Ray Spectra Constrain Axion-Like Particles},
author = {Joseph P. Conlon and M. C. David Marsh and Andrew J. Powell},
journal= {arXiv preprint arXiv:1509.06748},
year = {2016}
}