Authors: Chintan Shah, Stepan Dobrodey, Sven Bernitt, René Steinbrügge, José R. Crespo López-Urrutia, Liyi Gu, Jelle Kaastra
First Author’s Institution: Max-Planck-Institut für Kernphysik
Status: Published in The Astrophysics Journal, open access.
Over the past several years a mysterious spectral line at 3.5 keV has been identified in deep exposures carried out by many X-ray observatories, including Chandra, XMM-Newton, Suzaku, and NuSTAR. With most known atomic emission candidates ruled out and no known astrophysical origin, a dark matter source was hypothesized as the culprit producing this line. Indeed, there exist some dark matter models that predict the monochromatic emission of photons as part of the process of dark matter decay. Most notably sterile neutrino dark matter, a model that proposes a fourth, weighty and ‘sterile‘, neutrino flavor as the dark matter, predicts the emission of a 3.5 keV photon in the decay of a 7 keV sterile neutrino particle. However, the tables listing atomic transitions and their corresponding energies are quite incomplete, and often based on theoretical models that can be unreliable. As such, any assumption that a spectral feature has a dark matter origin, that is based mostly on its absence in tables of spectral data, is tenuous at best.
As a solution to this problem the authors attempt to produce the 3.5 keV line in the laboratory, motivated by a theoretical prediction of Liyi Gu, Jelle Kaastra, et al. (2016), wherein 
Figure 1: Schematic drawing of an electron beam ion trap. The electrons emitted at the cathode are forced to collide with 
The authors utilize an electron beam ion trap, a device that both produces and then confines highly charged ions. Electrons are accelerated from the cathode towards the 
Figure 2: Astrophysical observations of the 3.5 keV line reported in 3 separate papers, compared to the contribution from charge exchange. Red solid lines are gaussians fit to the underlying X-ray data in black. Blue and purple curves are the modeled data from the authors, produced using two slightly different modelling techniques. (Figure 6 in the paper)
The authors found strong evidence that decay from a highly excited state of the Sulfur ions, occupied due to charge exchange between the Sulfur ions and a neutral gas, can produce a spectral feature at 3.47 ± 0.06 keV, which is very close to the claimed lines found around 3.5 keV in astrophysical X-ray spectra. Furthermore they find other X-ray lines are also produced in the charge exchange process. Figure 2 shows a comparison between their modeled data and residuals from X-ray observations of the astrophysical line reported in 3 different papers. While inspecting Figure 2 one may wonder why the lower-energy spectral feature at ~3.15 keV predicted by the models is not present in the X-ray observations from the observatories. It turns out that there is another, much stronger, transition (
While everyone wants to find dark matter, it will be a long hard road. Today’s paper provides some evidence that one of the more recent intriguing hints could just be previously unknown astrochemistry. Further data analysis and targeted searches in dark matter rich and gas poor regions will be needed to close the issue. Hopefully the launch of the very high resolution x-ray observatory XARM in 2021 will be able to resolve many of the competing spectral lines from each other and put the matter to rest.
