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 $\gtrsim 12$ 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.
