Long-Exposure NuSTAR Constraints on Decaying Dark Matter in the Galactic Halo

We present two complementary NuSTAR x-ray searches for keV-scale dark matter decaying to monoenergetic photons in the Milky Way halo. In the first, we utilize the known intensity pattern of unfocused stray light across the detector planes-the dominant source of photons from diffuse sources-to separate astrophysical emission from internal instrument backgrounds using -7-Ms/detector deep blank -sky exposures. In the second, we present an updated parametric model of the full NuSTAR instrument background, allowing us to leverage the statistical power of an independent-20-Ms/detector stacked exposures spread across the sky. Finding no evidence of anomalous x-ray lines using either method, we set limits on the active-sterile mixing angle sin2(2 theta thorn for sterile-neutrino masses 6-40 keV. The first key result is that we strongly disfavor a-7-keV sterile neutrino decaying into a 3.5-keV photon. The second is that we derive leading limits on sterile neutrinos with masses-15-18 keV and-25-40 keV, reaching or extending below the big bang nucleosynthesis limit. In combination with previous results, the parameter space for the neutrino minimal standard model is now nearly closed.