Wednesday 19 November 2014

Update on the bananas

One of the most interesting physics stories of this year was the discovery of an unidentified 3.5 keV x-ray  emission line from galactic clusters. This so-called bulbulon can be interpreted as a signal of a sterile neutrino dark matter particle decaying into an active neutrino and  a photon. Some time ago I wrote about the banana paper that questioned the dark matter origin of the signal. Much has happened since, and I owe you an update. The current experimental situation is summarized in this plot:

To be more specific, here's what's happening.

  •  Several groups searching for the 3.5 keV emission have reported negative results. One of those searched for the signal in dwarf galaxies, which offer a  much cleaner environment allowing for a more reliable detection. No signal was found, although the limits do not exclude conclusively the original bulbulon claim. Another study looked for the signal in multiple galaxies. Again, no signal was found, but this time the reported limits are in severe tension with the sterile neutrino interpretation of the bulbulon. Yet another study failed to find the 3.5 keV line in  Coma, Virgo and Ophiuchus clusters, although they detect it in the Perseus cluster. Finally, the banana group analyzed the morphology of the 3.5 keV emission from the Galactic center and Perseus and found it incompatible with dark matter decay.
  • The discussion about the existence of the 3.5 keV emission from the Andromeda galaxy is  ongoing. The conclusions seem to depend on the strategy to determine the continuum x-ray emission. Using data from the XMM satellite, the banana group fits the background in the 3-4 keV range  and does not find the line, whereas this paper argues it is more kosher to fit in the 2-8 keV range, in which case the line can be detected in exactly the same dataset. It is not obvious who is right, although the fact that the significance of the signal depends so strongly on the background fitting procedure is not encouraging. 
  • The main battle rages on around K-XVIII (X-n stands for the X atom stripped of n-1 electrons; thus, K-XVIII is the potassium ion with 2 electrons). This little bastard has emission lines at 3.47 keV and 3.51 keV which could account for the bulbulon signal. In the original paper, the bulbuline group invokes a model of plasma emission that allows them to constrain  the flux due to the K-XVIII emission from  the  measured ratios of the strong S-XVI/S-XV and Ca-XX/Ca-XIX lines. The banana paper argued that the bulbuline model is unrealistic as it  gives inconsistent predictions for some plasma line ratios. The bulbuline group pointed out that the banana group used wrong numbers to estimate the line emission strenghts. The banana group maintains that their conclusions still hold when the error is corrected. It all boils down to the question whether the allowed range for the K-XVIII emission strength assumed by the bulbine group is conservative enough. Explaining the 3.5 keV feature solely by K-XVIII requires assuming element abundance ratios that are very different than the solar one, which may or may not be realistic.   
  •  On the other hand, both groups have converged on the subject of chlorine. In the banana  paper it  was pointed out that the 3.5 keV line may be due to the Cl-XVII (hydrogen-like chlorine ion) Lyman-β transition which happens to be at 3.51 keV. However the bulbuline group subsequently derived limits on the corresponding Lyman-α line at 2.96 keV. From these limits, one can deduce in a fairly model-independent way that the contribution of Cl-XVII Lyman-β transition is negligible.   

To clarify the situation we need more replies to comments on replies, and maybe also  better data from future x-ray satellite missions. The significance of the detection depends, more than we'd wish, on dirty astrophysics involved in modeling the standard x-ray emission from galactic plasma. It seems unlikely that the sterile neutrino model with the originally reported parameters will stand, as it is in tension with several other analyses. The probability of the 3.5 keV signal being of dark matter origin is certainly much lower than a few months ago. But the jury is still out, and it's not impossible to imagine that more data and more analyses will tip the scales the other way.

Further reading: how to protect yourself from someone attacking you with a banana.

Saturday 8 November 2014

Weekend Plot: Fermi and 7 dwarfs

This weekend the featured plot is borrowed from the presentation of Brandon Anderson at the symposium of the Fermi collaboration last week:

It shows the limits on the cross section of dark matter annihilation into b-quark pairs derived from gamma-ray observations of satellite galaxies of the Milky Way. These so-called dwarf galaxies are the most dark matter dominated objects known, which makes them a convenient place to search for dark matter. For example, WIMP dark matter annihilating into charged standard model particles would lead to an extended gamma-ray emission that could be spotted by the Fermi space telescope. Such emission coming from dwarf galaxies would be a smoking-gun signature of dark matter annihilation, given the relatively low level of dirty astrophysical backgrounds there (unlike in the center of our galaxy). Fermi has been looking for such signals, and a year ago they already published limits on the cross-section of dark matter annihilation into different final states.  At the time, they also found a ~2 sigma excess that was intriguing, especially in conjunction with the observed gamma-ray excess from the center of our galaxy. Now Fermi is coming back with an updated analysis using more data and better calibration. The excess is largely gone and, for the bb final state, the new limits  (blue) are 5 times stronger than the previous ones (black). For the theoretically favored WIMP annihilation cross section (horizontal dashed line), dark matter particle  annihilating into b-quarks is excluded if its mass is below ~100 GeV. The new limits are in tension with the dark matter interpretation of the galactic center excess (various colorful rings, depending who you like). Of course, astrophysics is not an exact science, and by exploring numerous uncertainties one can soften the tension. What is more certain is that a smoking-gun signature of dark matter annihilation in dwarf galaxies is unlikely to be delivered in the foreseeable future.