Galaxy cluster X-rays as a window to the dark sector
David Marsh (University of Oxford)

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 (3*10^19 m) scales. In this talk, I will discuss how the conversion of ALPs into X-ray photons in clusters of galaxies provide a window to the dark sector, and in particular how the different observations (null and otherwise) of an X-ray line at 3.5 keV from galaxies and clusters are consistent with a scenario in which dark matter decays into ALPs that subsequently convert into photons in astrophysical magnetic fields. Furthermore, I will mention how a 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.