New Constraints on Sterile Neutrino Dark Matter from $NuSTAR$ M31 Observations

We use a combined 1.2 Ms of NuSTAR observations of M31 to search for x-ray lines from sterile neutrino dark matter decay. For the first time in a NuSTAR analysis, we consistently take into account the signal contribution from both the focused and unfocused fields of view. We also reduce the modeling systematic uncertainty by performing spectral fits to each observation individually and statistically combining the results, instead of stacking the spectra. We find no evidence of unknown lines, and thus derive limits on the sterile neutrino parameters. Our results place stringent constraints for dark matter masses greater than or similar to 12 keV, which reduces the available parameter space for sterile neutrino dark matter produced via neutrino mixing (e.g., in the nu MSM) by approximately one-third. Additional NuSTAR observations, together with improved low-energy background modeling, could probe the remaining parameter space in the future. Lastly, we also report model-independent limits on generic dark matter decay rates and annihilation cross sections.