Dark Matter from the Sun?

This afternoon while I was struggling to pay attention during one of the presentations at the conference I’m at, when I noticed a potentially interesting story going around on Twitter. A little bit of research revealed that it relates to a paper on the arXiv, with the title Potential solar axion signatures in X-ray observations with the XMM-Newton observatory by Fraser et al. The first author of this paper was George Fraser of the University of Leicester who died the day after it was submitted to Monthly Notices of the Royal Astronomical Society. The paper has now been accepted and the final version has appeared on the arXiv in advance of its publication on Monday. The Guardian has already run a story on it.

This is the abstract:

The soft X-ray flux produced by solar axions in the Earth’s magnetic field is evaluated in the context of ESA’s XMM-Newton observatory. Recent calculations of the scattering of axion-conversion X-rays suggest that the sunward magnetosphere could be an observable source of 0.2-10 keV photons. For XMM-Newton, any conversion X-ray intensity will be seasonally modulated by virtue of the changing visibility of the sunward magnetic field region. A simple model of the geomagnetic field is combined with the ephemeris of XMM-Newton to predict the seasonal variation of the conversion X-ray intensity. This model is compared with stacked XMM-Newton blank sky datasets from which point sources have been systematically removed. Remarkably, a seasonally varying X-ray background signal is observed. The EPIC count rates are in the ratio of their X-ray grasps, indicating a non-instrumental, external photon origin, with significances of 11(pn), 4(MOS1) and 5(MOS2) sigma. After examining the constituent observations spatially, temporally and in terms of the cosmic X-ray background, we conclude that this variable signal is consistent with the conversion of solar axions in the Earth’s magnetic field. The spectrum is consistent with a solar axion spectrum dominated by bremsstrahlung- and Compton-like processes, i.e. axion-electron coupling dominates over axion-photon coupling and the peak of the axion spectrum is below 1 keV. A value of 2.2e-22 /GeV is derived for the product of the axion-photon and axion-electron coupling constants, for an axion mass in the micro-eV range. Comparisons with limits derived from white dwarf cooling may not be applicable, as these refer to axions in the 0.01 eV range. Preliminary results are given of a search for axion-conversion X-ray lines, in particular the predicted features due to silicon, sulphur and iron in the solar core, and the 14.4 keV transition line from 57Fe.

The paper concerns a hypothetical particle called the axion and I see someone has already edited the Wikipedia page to mention this new result. The idea of the axion has been around since the 1970s, when its existence was posited to solve a problem with quantum chromodynamics, but it was later realised that if it had a mass in the correct range it could be a candidate for the (cold) dark matter implied to exist by cosmological observations. Unlike many other candidates for cold dark matter, which experience only weak interactions, the axion feels the electromagnetic interaction, despite not carrying an electromagnetic charge. In particular, in a magnetic field the axion can convert into photons, leading to a number of ways of detecting the particle experimentally, none so far successful. If they exist, axions are also expected to be produced in the core of the Sun.

This particular study involved looking at 14 years of X-ray observations in which there appears to be an unexpected seasonal modulation in the observed X-ray flux which could be consistent with the conversion of axions produced by the Sun into X-ray photons as they pass through the Earth’s magnetic field. Here is a graphic I stole from the Guardian story:


Conversion of axions into X-rays in the Earth’s magnetic field. Image Credit: University of Leicester

I haven’t had time to do more than just skim the paper so I can’t comment in detail; it’s 67 pages long. Obviously it’s potentially extremely exciting but the evidence that the signal is produced by axions is circumstantial and one would have to eliminate other possible causes of cyclical variation to be sure. The possibilities that spring first to mind as an alternatives to the axion hypothesis relate to the complex interaction between the solar wind and Earth’s magnetosphere. However, if the signal is produced by axions there should be characteristic features in the spectrum of the X-rays produced that would appear be very difficult to mimic. The axion hypothesis is therefore eminently testable, at least in principle, but current statistics don’t allow these tests to be performed. It’s tantalising, but if you want to ask me where I’d put my money I’m afraid I’d probably go for messy local plasma physics rather than anything more fundamental.

It seems to me that this is in some sense a similar situation to that of BICEP2: a potentially exciting discovery, which looks plausible, but with alternative (and more mundane) explanations not yet definitively ruled out. The difference is of course that this “discovery paper” has been refereed in the normal way, rather than being announced at a press-conference before being subjected to peer review…

20 Responses to “Dark Matter from the Sun?”

  1. This isn’t really my field, but there’s obviously been a fair bit of discussion of this paper in our department recently so I’ve got a bit of a head-start when it comes to ploughing through all 67 pages.

    XMM-Newton’s highly eccentric orbit traces a very complex path through the Earth’s magnetosphere, and this results in a unique temporal signature in the X-ray background. The paper makes a very convincing argument that *something* from the Sun is interacting with the Earth’s magnetic field to produce ~keV X-rays. The spectrum of this anomalous X-ray background (Fig.14a in the paper), and its seasonal variation, both seem to be consistent with the axion hypothesis.

    My take is that it’s not an unambigious detection, but it’s very suggestive. It’s also an amazingly careful and painstaking piece of work, to tease out such a weak signal from 15+ years of archival data.

    PS. One minor point – the paper was actually posted to arXiv in March (when it was submitted). However, the press release has been timed to coincides with the (post-peer review) publication in MNRAS next week.

  2. i’ll bet $10 against one penny that it is another in a very long string of false positives for true believers in particle dark matter.

    Any takers?

    • telescoper Says:

      It may well be a false positive, but it’s a testable hypothesis. You know, like science.

      • True, but the claim that theoretical physicists have lost their way badly regarding the dark matter problem is also a testable hypothesis. You know, like science.

      • telescoper Says:

        Absence of evidence is not evidence of absence.

      • But from the point of Bayesian reasoning, shouldn’t probabilities be adjusted in light of scores (hundreds?) of negative empirical results?

        Or is it the permanent prior that there *must* be a pot of DM particles at the end of the rainbow?

        Are we in denial here?

      • Anton Garrett Says:

        I’m not sure *exactly* what hypothesis is in play in the immediately preceding two posts, and I think it matters.

  3. Andy Read on Radio 4 Today this morning http://www.bbc.co.uk/programmes/b04l3nnm at 48:30 minutes in

  4. Anton Garrett Says:

    “the axion feels the electromagnetic interaction, despite not carrying an electromagnetic charge”

    Any chance of a quick explanation of that? Presumably to do with coupling of emag and weak forces?

    • telescoper Says:

      Well the photon has no charge but mediates the electromagnetic interaction between charged particles. In standard QED at leading order photons do not couple directly to each other (ie there are no vertices in Feynman diagrams with eg 3 photons, only 2 charged particles and 1 photon, which is very restrictive). In weak interactions, the three bosons W+ W- and Z0 can interact at a single vertex. Introducing the axion (which couples directly to the photon) makes EM more similar to Weak interactions, ie more flexible in terms of what is conserved.

  5. Bryn Jones Says:

    Why do the authors believe the Sun will radiate axions? Or do interactions between axions and baryonic matter in the Sun reduce energies of dark matter axions passing through the Sun to something that will allow observable X-rays to be produced in the vicinity of the Earth?

    • telescoper Says:

      Axions (if they exist) will be produced in nuclear interactions just as photons are..

      • Bryn Jones Says:

        Ah. I see. The authors were hoping to test the existence of axions by indirectly detecting axions newly created in the Sun. Then if axions exist, axions might (or might not) also constitute dark matter. So they were not directly looking for dark matter particles.

        The media reports had confused me into thinking they were looking for direct evidence of dark matter particles.

        Thanks for explaining that.

      • telescoper Says:

        Indeed. Even if this “detection” stands it’s not obvious the axion would account for dark matter.

        In the same way the LHC might produce evidence for the existence of supersymmetric particles but their existence alone does not mean they would work as dark matter; that depends on their mass and cross-section.

      • Bryn Jones Says:

        Yes, absolutely.

  6. Would you mind explaining why the image posted doesn’t violate conservation of momentum? It looks like the axions are converting to X-Rays at an angle to their original path, but I thought that the conversion had to be co-linear, since no other particles are being produced.

    • What you can always ask when astrophysics is involved: Have you considered the effects of magnetic fields? In this case, maybe the “missing” momentum comes from the magnetic field.

      • Nick Kaiser Says:

        It seems to me SPV raises a very good question. The interaction is photon + axion -> photon where the initial photon comes from the magnetic field of the Earth. But (in our frame) that input photon has extremely low momentum (as the wavelength ~ thousands of km) so I’d have thought the input axion and outgoing x-ray photon momenta would be parallel. I must be missing something.

      • telescoper Says:


        You are right about momentum conservation. The authors of the paper argue that there is scattering of the axions before the interaction with magnetic field, but it seems to me that this has to produce a consistent 90-degree deflection and that doesn’t seem likely to me. Or am I missing something?


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