The Curious Case of the 3.5 keV “Line” in Cluster Spectra

Earlier this week I went to a seminar. That’s a rare enough event these days given all the other things I have to do. The talk concerned was by Katie Mack, who was visiting the Astronomy Centre and it contained a nice review of the general situation regarding the constraints on astrophysical dark matter from direct and indirect detection experiments. I’m not an expert on experiments – I’m banned from most laboratories on safety grounds – so it was nice to get a review from someone who knows what they’re talking about.

One of the pieces of evidence discussed in the talk was something I’ve never really looked at in detail myself, namely the claimed evidence of an  emission “line” in the spectrum of X-rays emitted by the hot gas in galaxy clusters. I put the word “line” in inverted commas for reasons which will soon become obvious. The primary reference for the claim is a paper by Bulbul et al which is, of course, freely available on the arXiv.

The key graph from that paper is this:


The claimed feature – it stretches the imagination considerably to call it a “line” – is shown in red. No, I’m not particularly impressed either, but this is what passes for high-quality data in X-ray astronomy!

There’s a nice review of this from about a year ago here which says this feature

 is very significant, at 4-5 astrophysical sigma.

I’m not sure how to convert astrophysical sigma into actual sigma, but then I don’t really like sigma anyway. A proper Bayesian model comparison is really needed here. If it is a real feature then a plausible explanation is that it is produced by the decay of some sort of dark matter particle in a manner that involves the radiation of an energetic photon. An example is the decay of a massive sterile neutrino – a hypothetical particle that does not participate in weak interactions –  into a lighter standard model neutrino and a photon, as discussed here. In this scenario the parent particle would have a mass of about 7keV so that the resulting photon has an energy of half that. Such a particle would constitute warm dark matter.

On the other hand, that all depends on you being convinced that there is anything there at all other than a combination of noise and systematics. I urge you to read the paper and decide. Then perhaps you can try to persuade me, because I’m not at all sure. The X-ray spectrum of hot gas does have a number of known emission features in it that needed to be subtracted before any anomalous emission can be isolated. I will remark however that there is a known recombination line of Argon that lies at 3.6 keV, and you have to be convinced that this has been subtracted correctly if the red bump is to be interpreted as something extra. Also note that all the spectra that show this feature are obtained using the same instrument – on the XMM/Newton spacecraft which makes it harder to eliminate the possibility that it is an instrumental artefact.

I’d be interested in comments from X-ray folk about how confident we should be that the 3.5 keV “anomaly” is real…

9 Responses to “The Curious Case of the 3.5 keV “Line” in Cluster Spectra”

  1. The link in this sentence, “There’s a nice review of this from about a year ago here which says this feature”, has a typo in it (missing r at the beginning). It should be

    Incidentally, the author seems to have used the phrase “astrophysical sigmas” before: .

  2. We should know one way or another next year thanks to Astro-H, which launches early 2016. The X-ray microcalorimeter instrument will measure the feature with a resolution sufficiently good to determine whether the line width is consistent with emission from a collisional plasma or from dark matter annihilation.

  3. philipmoriarty Says:

    Reminds me of the type of feature you see in an X-ray absorption or photoemission spectrum of a dilute sample acquired at a synchrotron beamline in the wee small hours. You start getting excited that there might be something there…

    …only to find that, after counting and integrating for eight hours, the ‘peak’ disappears into the background.

    • telescoper Says:

      I once went to an entire meeting on X-ray cosmology. I’ve been deeply sceptical about the entire field ever since..

  4. The astrophysical sigma could be the physical sigma multiplied by a normalized newsworthiness. I am specifically not mentioning Bicep. More likely, though, is that it is an estimate-by-eye of the physical sigma. Experienced spectroscopists do have an uncanny ability to pick out the funny bits in their data.

  5. Johannes Buchner Says:

    It’s now possible to do Bayesian model comparison of X-ray spectra using BXA

    It combines X-ray spectral analysis tools with the MultiNest algorithm.

  6. […] a year ago I wrote a blog post about a mysterious “line” in the X-ray spectra of galaxy clusters corresponding to an […]

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