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 galaxy clusters, 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. I will present the tightest constraints to-date on such ALPs, which we inferred using a deep Chandra grating observation of the extraordinarily luminous radio-quiet cluster-hosted quasar H1821+643.

Presentation

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