"Galaxy cluster X-rays as a window to the dark sector"

Axions and more general axion-like particles (ALPs) can convert into photons in the presence of coherent background magnetic fields. Galaxy clusters are the largest gravitationally bound objects in the universe and support magnetic fields that are coherent over kiloparsec scales. In this talk, I will discuss how the interconversion of light ALPs and X-ray photons in galaxy clusters provide a window to the dark sector, with several possible observational signals. Axion-photon conversion produces features in the thermal X-ray spectrum of the intra-cluster medium, and existing, archived X-ray data can be used to search for such a particle or, alternatively, to derive strong bounds on the axion-photon coupling. Furthermore, the different observations (null and otherwise) of an X-ray line at 3.5 keV from galaxies and galaxy clusters are consistent with a scenario in which dark matter decays into ALPs that subsequently convert into photons in astrophysical magnetic fields. Finally, I will mention how a cosmic background of relativistic ALPs is a generic outcome in known classes of stabilised string compactifications, and how such “dark radiation”, if present, may explain the longstanding question of the nature of the soft X-ray excess in clusters of galaxies.