Constraints on axion-like particles from X-ray observations of Hydra galaxy cluster

Abstract: Axion-like particles (ALPs) belong to a class of new pseudoscalar particles that generically couple to photons, opening the possibility of oscillations from photons into ALPs in an external magnetic field. Having witnessed the turbulence of their magnetic fields, these oscillations are expected to imprint irregularities in a limited energy range of the spectrum of astrophysical sources. In this study, Chandra observations of the Hydra galaxy cluster are used to constrain the value of the coupling of ALPs to photons. We consider the conversion of X-ray photons from the central source Hydra A in the magnetic field of the cluster. The magnetic field strength and structure are well determined observationally, which adds to the robustness of the analysis. The absence of anomalous irregularities in the X-ray spectrum of Hydra A conservatively provides the most competitive constraints on the coupling constant for ALP masses below 7e-12 eV at the level of g < 8.3e-12 GeV-1 at the 95% confidence level. Because of the specific phenomenology involved, these constraints actually hold more generally for very light pseudo-Nambu-Goldstone bosons.