Axionlike particles (ALPs) and photons interconvert 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 steplike 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 ${10}^{11}\mathrm{GeV}<M\lesssim {10}^{12}\mathrm{GeV}$ light ALPs give rise to interesting and unique observational signatures that may be probed by existing and upcoming x-ray (and potentially x-ray polarization) observations of galaxy clusters.

• Received 30 October 2015

DOI:https://doi.org/10.1103/PhysRevD.93.123526