Denis Wouters, Pierre Brun
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. These oscillations are expected to imprint irregularities in a limited energy range of the spectrum of astrophysical sources, witnessing the turbulence of their magnetic fields. In this study, Chandra observations of Hydra galaxy cluster are used to constrain the value of the coupling of ALPs to photons. The case of the conversion of X-ray photons from the central source Hydra A in the magnetic field of the cluster is considered. The magnetic field strength and structure is 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< 7.3e-12 GeV^{-1}. Because of the specific phenomenology involved these constraints actually hold more generally for very light pseudo Nambu-Goldstone bosons.
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http://arxiv.org/abs/1304.0989
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