## New Publication at the Open Journal of Astrophysics!

Posted in Open Access, The Universe and Stuff with tags , , , , , , , , on February 27, 2019 by telescoper

It’s nice to be able to announce that the Open Journal of Astrophysics has just published another paper. Here it is!

It’s by Ben Maughan of the University of Bristol (UK) and Thomas Reiprich of the University of Bonn (Germany). You can find the accepted version on the arXiv here.

This is the first paper we have published in the section called High Energy Astrophysical Phenomena.

Thanks to the Editor and referees for dealing with this one so efficiently!

We have a few other papers coming up for publication soon, and some have been sent back to authors for revise and resubmit so we will almost certainly have further announcements to make soon.

P.S. Nobody spotted that I put the wrong DOI on the front page. I did that deliberately to see who was paying attention. Anyway, I’ve now put the right one on.

## The 3.5 keV “Line” that (probably) wasn’t…

Posted in Bad Statistics, The Universe and Stuff with tags , , , , , , , on July 26, 2016 by telescoper

About a year ago I wrote a blog post about a mysterious “line” in the X-ray spectra of galaxy clusters corresponding to an energy of around 3.5 keV. 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!

Anyway, there has just appeared on the arXiv a paper by the Hitomi Collaboration describing what are basically the only set of science results that the Hitomi satellite managed to obtain before it fell to bits earlier this year. These were observations of the Perseus Cluster.

Here is the abstract:

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified E=3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported flux level. The high flux derived with XMM MOS for the Perseus region covered by Hitomi is excluded at >3-sigma within the energy confidence interval of the most constraining previous study. If XMM measurement uncertainties for this region are included, the inconsistency with Hitomi is at a 99% significance for a broad dark-matter line and at 99.7% for a narrow line from the gas. We do find a hint of a broad excess near the energies of high-n transitions of Sxvi (E=3.44 keV rest-frame) – a possible signature of charge exchange in the molecular nebula and one of the proposed explanations for the 3.5 keV line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.

And here is the killer plot:

The spectrum looks amazingly detailed, which makes the demise of Hitomi all the more tragic, but the 3.5 keV is conspicuous by its absence. So there you are, yet another supposedly significant feature that excited a huge amount of interest turns out to be nothing of the sort. To be fair, as the abstract states, the anomalous line was only seen by stacking spectra of different clusters and might still be there but too faint to be seen in an individual cluster spectrum. Nevertheless I’d say the probability of there being any feature at 3.5 keV has decreased significantly after this observation.

P.S. rumours suggest that the 750 GeV diphoton “excess” found at the Large Hadron Collider may be about to meet a similar fate.

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

Posted in Bad Statistics, The Universe and Stuff with tags , , , , , , on July 22, 2015 by telescoper

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…

## The Biggest Things in the Universe

Posted in The Universe and Stuff with tags , , , , on November 12, 2011 by telescoper

I’ve never really thought of this blog as a vehicle for promoting my own research in cosmology, but it’s been a while since I posted anything very scientific so I thought I’d put up a brief advertisement for a paper that appeared on the arXiv this week by myself and Ian Harrison (who is a PhD student of mine). Here is the abstract, which I think is pretty informative about the contents of the paper; would that were always the case!

Motivated by recent suggestions that a number of observed galaxy clusters have masses which are too high for their given redshift to occur naturally in a standard model cosmology, we use Extreme Value Statistics to construct confidence regions in the mass-redshift plane for the most extreme objects expected in the universe. We show how such a diagram not only provides a way of potentially ruling out the concordance cosmology, but also allows us to differentiate between alternative models of enhanced structure formation. We compare our theoretical prediction with observations, placing currently observed high and low redshift clusters on a mass-redshift diagram and find – provided we consider the full sky to avoid a posteriori selection effects – that none are in significant tension with concordance cosmology.

The background to this paper is that,  according to standard cosmological theory, galaxies and other large-scale structures such as galaxy clusters form hierarchically. That is to say that they are built from the bottom-up from a population of smaller objects that progressively merge  into larger and larger structures as the Universe evolves. At any given time there is a broad distribution of masses, but the average mass increases as time goes on. Looking out into the distant Universe we should therefore see fewer high-mass objects at high redshift than at low redshift.

Recent observations – I refer you to our paper for references – have revealed evidence for the existence of some very massive galaxy clusters at redshifts around unity or larger, which corresponds to a look-back time of greater than 7 Gyr. Actually these are not at high redshift compared to galaxies, which have bee found at redshifts around 10, where the lookback time is more like 12 Gyr, but these are at least a thousand times less massive than large clusters so their existence in the early Universe is not surprising in the framework of the standard cosmological model. On the other hand, clusters of the masses we’re talking about – about 1,000,000,000,000,000 times the mass of the Sun – should form pretty late in cosmic history so have the potential to challenge the standard theory.
In the paper we approach the issue in a different manner to other analyses and apply Extreme Value Statistics to ask how massive we would expect the largest cluster in the observable universe should be as a function of redshift. If we see one larger than the limits imposed by this calculation then we really need to consider modifying the standard theory. This way of tackling the problem attempts to finesse a  number of biases  in the usual approach, which is to attempt to estimate the number-density $n(M)$ of clusters as a function of mass $M$, because it does not require a correction for a posteori  selection effects; it is not obvious, for example, prevcisely what volume is being probed by the surveys yielding these cluster candidates.

Anyway, the results are summarised in our Figure 1, which shows some estimated cluster masses, together with their uncertainties, superimposed on the theoretical distribution of the mass of the most massive cluster at that redshift:

If you’re wondering why the curves turn down at very low redshift, it’s just because the volume available to be observed at low redshift is small: although objects are generally more massive at low redshift, the chance of getting a really big one is reduced by the fact that one is observing a much smaller part of space-time.

The results show:  (a) that, contrary to some claims, the current observations are actually entirely consistent with the standard concordance model; but also  (b)  that the existence of clusters at redshifts around 1.5 with masses much bigger than $10^{15} M_{\odot}$ would require the tabling of an amendment to the standard theory.

Of course this is is a very conservative approach and it yields what is essentially a null result, but I take the view that while theorists should be prepared to consider radical new theoretical ideas, we should also be conservative when it comes to the interpretation of data.