Observational evidence for axion(-like) particles

Several unexpected astrophysical observations can be explained by gravitationally captured massive axions or axion-like particles produced inside the Sun or other stars. Their radiative decay in solar outer space would give rise to a ‘self-irradiation’ of the whole star, providing the missing corona heating source. In analogy with the Sun-irradiated Earth atmosphere, the temperature and density gradient in the corona−chromosphere transition region is suggestive for an omnipresent irradiation of the Sun, which is the strongest evidence for the generic axion-like scenario. The radiative decay of a population of such elusive particles mimics a hot gas. The recently reconstructed quiet solar X-ray spectrum supports this work, since it covers the expected energy range, and it is consistent with the result of a simulation based on Kaluza–Klein axions above ∼1 keV. At lower energies, using also a ROSAT observation, only ∼3% of the solar X-ray intensity is explained. Data from orbiting X-ray Telescopes provide upper limits for particle decay rates 1 AU from the Sun, and suggest new types of searches on Earth or in space. In particular, X-ray observatories, with an unrivalled equivalent fiducial volume of ∼103 m3 for the 0.1–10 keV range, can search for the radiative decay of new particles even from existing data.