Of Particular Significance

X-Rays From Dark Matter? A Little Hint For You To Enjoy

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 02/18/2014

Well it’s not much to write home about, and I’m not going to write about it in detail right now, but the Resonaances blog has done so (and he’s asking for your traffic, so please click):

A team of six astronomers reports that when they examine the light (more specifically, the X-rays) coming from clusters of galaxies around the sky, and account for all the X-ray emission lines [light emitted in extremely narrow bands by atoms or their nuclei] they know about, there’s an excess of photons [particles of light] with energy E=(3.55-3.57)+/-0.03 keV, a “weak unidentified emission line”, that can’t easily be explained.  What could it be?

[A keV is 1000 eV; an eV is an electron-volt, an amount of energy typical of chemical reactions.  Note that physicists and astronomers commonly use the word “light” to refer not just to “visible light” — the light you can see — but to all electromagnetic waves, no matter what their frequency. ]

Well first: is this emission line really there?  The astronomers claim to detect it in several ways, but “the detection is at the limit of the current instrument capabilities and subject to significant modeling uncertainties” — in other words, it requires some squinting — so they are cautious in their statements.

Second: if it’s really there, what’s it due to?  Well, the most exciting and least likely possibility is that it’s from dark matter particles decaying to a photon with the above-mentioned energy plus a second, unobserved, particle — perhaps a neutrino, perhaps something else.   I’ll let Resonaances explain the sterile neutrino hypothesis, in which the dark matter particles are kind of like neutrinos — they’re fermions, like neutrinos, and they are connected to neutrinos in some way, though they aren’t as directly affected by the weak nuclear force.

But before you get excited, note that the authors state: “However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model, significantly deviating from other subsamples.”  In other words: don’t get excited, because something very funny is going on in the Perseus cluster, and until that’s understood, the data can’t be said to be particularly consistent with a dark matter hypothesis.

One more anomaly — one more hint of dark matter — to put on the pile of weak and largely unrelated hints that we’ve already got!  I don’t suggest losing sleep over it… at least not until it’s confirmed by other groups and the Perseus cluster’s odd emissions are explained.

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15 Responses

  1. The Boyarsky paper is both redundant and a cross check. They claim to see it in Perseus, but also M31 (which the Bulbul group didn’t study). The Bulbul paper uses XMM but two instruments on it (PN and MOS), which separately see the line (in the full analysis) and use Chandra data to confirm the Perseus line.

    Still, definitely one of those “need much more data” things…

    1. Thanks, Neal! [Neal Weiner is one of the world’s experts on the particle physics possibilities for dark matter.] Any thoughts on Finkbeiner et al’s latest paper?

  2. I can’t help getting excited about this, especially since another group is reporting the same line: http://arxiv.org/abs/1402.4119
    However both groups use the XMM-Newton observatory (though the analysis cited in the article uses Chandra to check). And if the line is real, it might still have a more boring non-DM explanation, I fear.

      1. It’s not the same data set, but I’m not sure whether there is any overlap. In 1402.4119 they state “Two observations on-center (ObsIDs 0305780101 and 0085110101) are not included in the analysis.”. These are exactly the two observations listed in 1402.2301 for the Perseus cluster, so it seems there is some independence. Maybe this is also the reason that 1402.4119 does not see the anomalously high flux from Perseus. Both analyses use the XMM instruments, but a straight-forward error from the instruments seems unlikely because of the per-cluster blueshift applied in the 1402.2301 analysis.

        From my complete outsider’s perspective the most likely way for this exciting hint to go away would be if someone finds a way how baryonic matter could create this line.

          1. The paper (1402.4119) lists three criteria for rejecting observations: 1) count rates deviate too much from the mean value, 2) F_in – F_out ratio too high, 3) MOS1 camera data before the loss of one of the CCDs is rejected (only applied for Andromeda, I think).

            1 and 2 aim to reduce the influence of solar events, I think. I’m not sure about the reason for 3.

            It is not clear to me which of these criteria (if any) caused the exclusion of the on-center Perseus observations.

  3. “… there’s an excess of photons [particles of light] with energy E=(3.55-3.57)+/-0.03 keV, a “weak unidentified emission line”, that can’t easily be explained. …”

    Reminds me of the “Roadrunner” cartoons where the roadrunner, once again, kills off the coyote the speeds away with such a large acceleration he leaves only a puff of smoke behind.

    Could these excessive photons be the residue left behind when black holes disintegrate visible matter down to dark matter? The jets we see firing out from center of galaxies (black holes) could also contain the dark matter as well.

    If this hypothesis is valid and the dark matter only reacts with fermions through the gravitational field then dark matter should be increasing and visible matter (fermions) should be decreasing. Are there any hints to this equation?

    As I mentioned in a previous comment, could black holes be chaotic orifices converting, (pulverizing, canceling the strong force), matter “down” to dark? How high (dense) must the gravitational field be to have enough energy to break up the strong force? Could this hypothesis explain the boundary conditions of a black hole?

  4. Matt: I did click and asked a question for which he gave a good answer! I would like to see your input also on this question.
    ———————————————————————————————– kashyap vasavada said…
    Current standard model has symmetry between no of quarks and no of leptons, except handedness in the case of neutrinos.Will the sterile neutrinos disturb this symmetry i.e now you will need more than 6 quarks? Are there already group structures which can accommodate sterile neutrinos and nothing else or they need a number of new particles?
    17 February 2014 16:16
    Jester said…
    You can just add sterile neutrinos alone. In the simplest version they are singlets under the SM gauge symmetry group so the local symmetry structure is unchanged. They violate lepton number if they have a Majorana mass, but that doesn’t lead to any conflict with current experimental data. They can also fit in the grand unification scheme, e.g. in SO(10) models.
    But now I am thinking it would be good to have sterile neutrinos with both left and right hand components! All other particles have right and left hand components and our known neutrinos could have non zero masses. So why not neutrinos sterile or not with both right and left handedness? Weak coupling may be a separate question. Also I am still not sure what this does to symmetry between quarks and leptons.

    1. Jester is a first-rate particle physicist and there’s nothing for me to add to his answer.

      As for your second question, you are not correct when you say:

      ” All other particles have right and left hand components and our known neutrinos could have non zero masses. ”

      First of all, it’s only the matter fermions (not the Higgs, W or Z) which have these components; and neutral fermions do not need to have both right- and left- components, even if they have a mass. The known neutrinos are only left-handed (and anti-neutrinos are only right-handed) and yet they have the “Majorana”-type masses that Jester mentions. Sterile neutrinos could, for example, be viewed as right-handed neutrinos. They too could have Majorana masses, and not require a left-handed version, just as neutrinos do not require a right-handed version. However, there are a number of other possibilities, including that the sterile neutrinos have Dirac masses.

      By the way, there is no symmetry or balance among quarks and leptons. Each quark comes in three versions, called “colors”, that are changed from one to another by the strong nuclear force. No such versions occur for leptons. And among the left-handed fermions are quarks, antiquarks, charged leptons, charged antileptons, neutrinos, but NOT anti-neutrinos. See http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/the-known-particles-if-the-higgs-field-were-zero/

      1. Thanks Matt.I am beginning to understand now what you and Jester are saying. However I always thought that the fact that there are exactly 6 quarks and exactly 6 leptons had some deeper reason. May be no one has found a group theoretical connection. Yes the fact of three colors for quarks is a complication! But are you saying that the existence of exactly 6 quarks and exactly 6 leptons, until they find sterile neutrinos, is an accident?

        1. It’s not an accident, but the structure of the accident is different than 6 = 6 . It’s more subtle than that. To understand this you have to study more advanced topics — anomalies, and the Lie algebras known as SU(5) and SO(10).

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