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

TL;DR

This paper uses galaxy cluster X-ray spectra to set new constraints on light axion-like particles, exploring their potential conversion signatures and observational implications in astrophysical environments.

Contribution

It introduces a novel method to constrain ALPs using thermal X-ray spectra from galaxy clusters, extending bounds into previously unexplored parameter regions.

Findings

Existing X-ray data can constrain ALPs with low masses and high coupling constants.

Conversion of photons into ALPs can produce detectable spectral features.

Upcoming X-ray observations could further probe ALP properties.

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 $m_a \lesssim 1 \times 10^{-12}\,{\rm eV}$ 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.