Abstract
Abstract 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 ∼10 3 m 3 for the 0.1–10 keV range, can search for the radiative decay of new particles even from existing data.
Highlights
The direct detection of dark-matter particles has proved elusive since the first gravitational observation of a non-luminous matter in the Universe
Considerations based on axions apply as well, invoking either its 2γ decay mode or its coherent conversion to a photon within astrophysical electric/magnetic fields via the Primakoff effect.1) It is worth noting an additional suggestion which fits the reasoning of this work
We argue that the photon decay of some hypothetical particles, we call them more generally ‘axion-like’, must be involved in certain unexplained astrophysical observations
Summary
In order to explain the widespread ionization of the interstellar medium in the Galaxy, speculations included the electromagnetic decay of real or hypothetical exotic particles [1], massive dark-matter neutrinos providing the emission of the required ionizing photon [2]. Following the reasoning of this work along with the mentioned observations, we suggest performing a specific axion search in a new type of experiment either on the surface (and in space) or underground, aiming to directly detect the 2γ decay mode. Following this axion scenario, some astrophysical measurements could be reconsidered or re-analysed. A slightly modified design in future X-ray space detectors could allow them to operate as sensitive orbiting axion telescopes
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