The current status of 3.5 keV line and sterile neutrino dark matter
Professor Alexey Boyarsky, Leiden University, The Netherlands
Professor Boyarsky will review the current status of the attempts to figure our the origin of the 3.5 kev line and of sterile neutrino as dark matter candidate.
Plasma models and charge exchange
Professor Jelle Kaastra, SRON and Leiden University, The Netherlands
Charge exchange (CX) is the process where an electron is transferred from one atom to another. Here Professor Kaastra considers CX between cold, neutral hydrogen gas and hot plasma. The CX process can give rise to line radiation in the X–ray and other spectral bands. The spectrum for this process is quite different from the normal X–ray emission spectrum from the hot plasma. CX with oxygen ions has been seen in many spectra from Solar System objects. Professor Kaastra shows that the CX process in clusters of galaxies occurring between the hot intracluster gas and cold gas filaments is very effective and produces significant line radiation near 3.5 keV due to interaction with sulphur. Additional evidence for this process in clusters comes from grating spectra of clusters in the oxygen band, as well as from UV spectroscopy. Professor Kaastra concludes this presentation with a brief summary of atomic physics, spectral modelling and calibration issues related to the 3.5 keV band.
X–ray probes of dark matter
Professor Tesla Jeltema, University of California, Santa Cruz, USA
Professor Jeltema will discuss the use of X–ray observations to constrain particle dark matter models. In particular, she will review searches for X–ray lines including observations of and constraints on the 3.5 keV line. She will also discuss the use of X–ray observations of galaxies and clusters to constrain dark matter self–interaction.
Laboratory X–ray astrophysics with highly charged ions
Dr Chintan Shah, Max Planck Institute for Nuclear Physics, Heidelberg, Germany
X–ray observations provide insight into the state and dynamics of a variety of astrophysical plasmas. Its interpretation relies on synthetic spectra which are based on plasma models. Those models, in turn, depend heavily on the accurate knowledge of the underlying atomic processes. A recent Hitomi observation of the Perseus cluster  provided an unprecedented high–resolution spectrum. It revealed not only well–resolved lines originating from most abundant highly charged ions, but also uncovered significant shortcomings of present spectral packages SPEX and AtomDB, namely inaccurate energies and atomic–scale processes completely missing from the models. For example, a weak line feature around 3.5 keV found in the spectra of galaxy clusters sparked tremendous interest when it was attributed to a possible dark matter decay process . However, charge exchange between bare sulphur ions and hydrogen, a process not included in spectral models, was found to be a more plausible explanation. This talk will report on laboratory electron beam ion trap experiments that compellingly supported charge–exchange mechanism for 3.5 keV X–ray line . It will further underpin how the incomplete knowledge of atomic processes limits the amount of information that can be extracted from current and next–generation X–ray satellites such as Athena and XRISM.
 Hitomi Collaboration, Nature 535, 117–121 (2016)
 E Bulbul et al, Astrophys. J 789, 13 (2014)
 C Shah et al, Astrophys. J 833, 52 (2016)
Sterile neutrino constraints from Chandra to eROSITA
Dr Florian Hofmann, Max Planck Institute for Extraterrestrial Physics (MPE), Germany
There is an ongoing debate on the nature of the 3.55 keV emission line in X-ray spectra of astronomical objects. One possible explanation is the decay of 7.1 keV sterile neutrinos, which have been proposed as a candidate for dark matter. The authors performed an independent search for the emission line in X-ray spectra of galaxy clusters obtained from the Chandra observatory data archive. They will also discuss future capabilities of the eROSITA observatory in the search for dark matter.