Almost) Closing the Sterile Neutrino Dark Matter Window with NuSTAR

We use NuSTAR observations of the Galactic Center to search for X-ray lines from the radiative decays of sterile neutrino dark matter. Finding no evidence of unknown lines, we set limits on the sterile neutrino mass and mixing angle. In most of the mass range 10-50 keV, these are now the strongest limits, at some masses improving upon previous limits by a factor of ~10. When combined with constraints on the primordial lepton asymmetry and structure formation, the allowed mass range of the the $\nu$MSM framework is reduced by more than half. Future NuSTAR observations may be able to cover much of the remaining parameter space.