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 θ ) 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.