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

Abstract

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 ∼ 1 keV), the non-cooling of oldest Stars, etc. A temperature of ∼ 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 ∼ 1 keV. The derived axion luminosity (L a ≈ 0.16L ) fits the cosmic energy density spectrum and is compatible within 2σ 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 ∼ 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 ∼ 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.

Overlooked Astrophysical Signatures of Axion(-Like) Particles

“…The values obtained with RHESSI in the ~3-12 keV range, for such extremely quiet periods at the present solar minimum phase, correlate with those from another orbiting X-ray solar telescope (GOES), implying a real signal in this actually not so low energy band [7,8] for a cool Star like our Sun. This we consider as additional supporting evidence for the (massive) solar axion scenario [2]. Because, a conventional explanation with electrons raises a new problem: How is such a population of energetic electrons created in the quiet corona?…”

“…In fact, the RHESSI mission has recently repeated several off-pointing observations above the solar limb during non-flaring, spotless and activeregion-free Sun, i.e., during quiet Sun conditions [7], arriving apparently at a hard X-ray spectrum, which is rather similar to the reconstructed one from YOHKOH measurements in the previous solar cycle minimum (see Figure 9 in ref. [2]). The values obtained with RHESSI in the ~3-12 keV range, for such extremely quiet periods at the present solar minimum phase, correlate with those from another orbiting X-ray solar telescope (GOES), implying a real signal in this actually not so low energy band [7,8] for a cool Star like our Sun.…”

“…If these less massive axions (see ref. [2]) build up a large trapped component around the Sun, their spontaneous decay could explain the low-energy X-rays from the quiet Sun. The induced "decay" in magnetic fields of light axions at the same lower energy range, created probably in the same place, comes in addition.…”

“…c) the so far ignored inner magnetic fields of the Sun giving eventually rise to the possible involvement also of other exotica like millicharged particles, etc., which are expected to interact very feebly with ordinary matter. Their coupling to the magnetic field can result to an accumulation over cosmic times, without the need to invoke the usually inefficient gravitational (self)trapping [2]. In the following, we give a few solar observations, which we have reproduced from published work or derived from public data of the Yohkoh mission.…”

“…Here we mainly discuss striking solar observations of unknown origin which seem to require the involvement of axions or other exotica with similar properties. Previous work [2][3][4] addressed a steady X-ray emission due to very few gravitationally trapped massive axion-like particles (about 1 in 10 7 , or only ~100 kg/s since 4.6 Gyears), giving rise to a self-irradiation of the whole Sun. In this work, we mainly argue in favour of a second solar X-ray component expected to originate from the widely accepted interaction of light axions or the like with the inner/outer solar magnetic fields, as they stream out of the Sun.…”