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

Understand

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.