  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 $m_a \lesssim 1 \times 10^{-12}\,{\rm eV}$ down to $M \sim
7\times 10^{11}\, {\rm GeV}$, and that for $10^{11}\, {\rm GeV} < M \lesssim
10^{12}$ 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.
