Observational evidence for gravitationally trapped massive axion(-like) particles

Several unexpected astrophysical observations can be explained by gravitationally captured massive axions or axion-like particles, which are produced inside the Sun or other stars and are accumulated over cosmic times. Their radiative decay in solar outer space would give rise to a `self-irradiation' of the whole star, providing the time-independent component of the corona heating source (we do not address here the flaring Sun). 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 same mechanism is compatible with phenomena like the solar wind, the X-rays from the dark-side of the Moon, the X-Ray Background Radiation, the diffuse X-ray excesses (below $\sim 1$ keV), the non-cooling of oldest Stars, etc. A temperature of $\sim 10^6$ K is observed in various places, while the radiative decay of a population of such elusive particles mimics a hot gas, which fits unexpected astrophysical X-ray observations. Furthermore, the recently reconstructed quiet solar X-ray spectrum during solar minimum 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 $\sim$ 1 keV. The derived axion luminosity ($L_a\approx 0.16L_{\odot}$) fits the cosmic energy density spectrum and is compatible within 2$\sigma $ with the recent SNO result, showing the important interplay between any exotic energy loss mechanism and neutrino production. At lower energies, using also a ROSAT observation, only $\sim 3\%$ of the 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 $\sim 10^3~m^3$ for the 0.1 - 10 keV range, can search for the radiative decay of new particles even from existing data. This work introduces the elongation angle of the X-ray Telescope relative to the Sun as a relevant new parameter.