Dark matter is one of the big mysteries of modern cosmology. We have lots of evidence to support its existence, but the enigmatic, invisible stuff has been notoriously resistant to direct detection. Because of this, there have been lots of ideas about just what dark matter might be made of, and just how such particles might be detected. One of these ideas focuses on particles known as axions, but recent work has eliminated at least some versions of axions as a dark matter candidate. 

Axions are a theoretical particle devised in the 1970s to address certain issues in quantum chromodynamics (a theory that describes the behavior of the strong force). According to the model, axions would be low mass, chargeless particles that don’t interact strongly with light, which sounds like a perfect dark matter candidate. While there have been attempts to observe the effect of axions, they haven’t been successful. New work in Physical Review Letters not only didn’t find evidence of axions, it places strong constraints on their existence.

Although axions don’t interact strongly with light, they can interact in subtle ways. By coupling with magnetic fields axions could decay into light, and they could also absorb gamma rays. So a team of astronomers combed through six years of gamma ray data from a galaxy known as Perseus A. The central region of this galaxy not only emits strong gamma rays, the region is known to have magnetic fields. If dark-matter axions exist, they should interact with the gamma rays, changing the spectrum from Perseus A. But the team found no evidence of any change in the spectrum. The results aren’t strong enough to rule out the existence of axions, but it is strong enough to rule them out as a source of dark matter.

So dark matter is still a mystery. We now have a better idea of what dark matter isn’t, but that still doesn’t tell us what it is.

Paper: M. Ajello et al. Search for Spectral Irregularities due to Photon–Axionlike-Particle Oscillations with the Fermi Large Area Telescope. Phys. Rev. Lett. 116, 161101 (2016)  arXiv:1603.06978 [astro-ph.HE]

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There’s been a few articles in the popular press recently announcing the discovery of dark matter particles coming from the Sun. This is not the case. No science team is claiming they’ve discovered dark matter. The story traces it origin to a paper being published in MNRAS, which could be an indirect detection of dark matter, but could also be a few other things. It is an interesting paper, nonetheless.

What the paper actually looks at is possible evidence of axions coming from the Sun. Axions are hypothetical particles that were first proposed in 1977 to address certain issues in quantum chromodynamics (QCD), which describes the behavior of quarks and such that make up protons and electrons (among other things). There has been no evidence for axions, but if they exist, then the Sun should produce axions through nuclear interactions similar to the way it produces neutrinos.

According to the model, axions would be be low mass, chargeless particles that don’t interact strongly with light. This would make them a suitable candidate for cold dark matter. Of course this would also make them particularly difficult to observe directly. But unlike other dark matter candidates, axions do interact slightly with light and electromagnetic fields. So we might be able to see them by their interaction with Earth’s magnetic field.

In the paper the authors demonstrate that high energy axions striking the Earth’s magnetic field could produce x-rays. When they modeled this idea they found that axion-induced x-rays would have a seasonal variation due to the varying tilt of Earth’s axis (and thus its magnetic field). They then looked at x-ray data from the XMM-Newton spacecraft, and found that it had in fact detected a seasonal variation of x-rays. This variation is consistent with the axion model.

Of course before we can say that it’s definitely axions, we need to look for other possible explanations. Not surprisingly there are several. For example, the seasonal variations could be due to subtle and complex interactions between the solar wind and Earth’s magnetic field. So axions are a good candidate for this cyclic variation, but not the only candidate.

Where things could get interesting is through a more detailed study of the seasonal x-ray variations. If they are produced by axions, then the x-rays should have distinct signatures in their spectrum that would distinguish them from other models. So like many cutting edge discoveries it has potential without clear confirmation.

Paper: G. W. Fraser, et al. Potential solar axion signatures in X-ray observations with the XMM-Newton observatory. Monthly Notices of the Royal Astronomical Society, 20 October, 2014

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