Tightening NuSTAR Constraints on keV-Scale Sterile Neutrinos: Present and Future

Sterile neutrinos with masses at the keV scale are a popular dark matter candidate, with a clear decay signature including a mono-energetic x-ray photon that can be investigated using existing x-ray telescopes. In particular, the large solid angle of the NuSTAR observatory for unfocused x-rays has led to world-leading sensitivity to the decays of sterile neutrinos in the mass range 10-50 keV, across a variety of astrophysical targets. I will describe our group's analysis of archival NuSTAR observations of the M31 galaxy (${\sim}$1.2 Ms) and dedicated high-latitude observations near the Galactic Center (${\sim}$190 ks); together, these reduce the size of the remaining parameter space for resonantly-produced sterile neutrinos by nearly one-third. Finally, I will discuss the impact of recent improvements in modeling the NuSTAR instrument background, particularly in the x-ray energy range 3-5 keV (sterile neutrino masses 6-10 keV), and the implications for future sterile-neutrino dark matter searches using NuSTAR data.