Abstract

Axion-Like Particles (ALPs) are a well-motivated extension to the Standard Model of particle physics. In the presence of a magnetised plasma such as the intracluster medium (ICM) in rich cool-core clusters of galaxies, light may inter-convert into ALPs of a given mass m as determined by the ALP -photon coupling strength. At X-ray energies, we would expect photon-ALP oscillations to induce energy-dependent modulations on the intrinsic spectra of central AGN as their quanta travel through the magnetised ICM . This can be used to place bounds on the ALP -photon coupling for very-light ALPs, that is, of log(m/eV) < -12, given a model for the ICM magnetic field. Using a cell-based approach for the latter, I will present the tightest constraints to-date on such ALPs, which we have inferred from a deep Chandra grating observation of the extraordinarily luminous radio-quiet cluster-hosted quasar H1821 +643. I will also present the exciting prospects for cluster-ALP studies given the unprecedented energy resolution of the next-generation X-ray telescope Athena.