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

Abstract

We present two complementary NuSTAR x-ray searches for keV-scale dark matter decaying to mono-energetic 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 $\sim$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 $\sim$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 ${\sin }^2(2\theta )$ for sterile-neutrino masses 6--40 keV. The first key result is that we strongly disfavor a $\sim$7-keV sterile neutrino decaying into a 3.5-keV photon. The second is that we derive leading limits on sterile neutrinos with masses $\sim$15--18 keV and $\sim$25--40 keV, reaching or extending below the Big Bang Nucleosynthesis limit. In combination with previous results, the parameter space for the Neutrino Minimal Standard Model ($\nu$MSM) is now nearly closed.

Comments: Minor changes to text/references to reflect published version, conclusions unchanged