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 12 keV, which reduces the available parameter space for sterile neutrino dark matter produced via neutrino mixing (e.g., in the νMSM) 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. I.via a small mixing with active neutrinos [12], which may be enhanced by the presence of primordial lepton asymmetry [13]. As the mixing angle determines both the abundance and decay rate, there is a finite window in the mass-mixing angle parameter plane in which sterile neutrinos could constitute the full DM abundance, thus allowing this scenario to be fully testable. Closing this window would imply additional physics and production mechanisms are needed to make sterile neutrinos a viable DM candidate [14][15][16][17][18][19][20][21]. The existence of sterile neutrino DM could provide strong clues for explaining neutrino mass and baryogenesis [22], such as the scenario advocated in the νMSM model [23][24][25][26].Due to several sensitive X-ray instruments, such as Chandra, Suzaku, XMM-Newton, and INTEGRAL, stringent constraints on X-ray line emission have been obtained using many different observations (e.g., Refs. [27][28][29][30][31][32][33]). Interest in these topics was heightened with the tentative detection of a 3.5-keV line from cluster observations [34], which was followed up by many observational studies . The nature of this line is still inconclusive. The line could be a signature of sterile neutrino DM [57] or other candidates [58][59][60][61][62]. However, as the line flux is weak, astrophysical modeling systematics [37,41] or new astrophysical processes [63,64] could also be the explanation. New detectors [44,56,65,66] or techniques, such as velocity spectroscopy [67,68], are likely required to fully determine its nature. (Recently, Ref. [69] claims that blank-sky observations with XMM-Newton disfavor the DM interpretation of the 3.5-keV line. On the other hand, Ref. [70] claims detection of the 3.5 keV line in the Milky Way halo up to 35 • with XMM-Newton, and arXiv:1901.01262v2 [astro-ph.HE]

We analyze two dedicated NuSTAR observations with exposure ∼190 ks located ∼10°from the Galactic plane, one above and the other below, to search for x-ray lines from the radiative decay of sterile-neutrino dark matter. These fields were chosen to minimize astrophysical x-ray backgrounds while remaining near the densest region of the dark matter halo. We find no evidence of anomalous x-ray lines in the energy range 5-20 keV, corresponding to sterile neutrino masses 10-40 keV. Interpreted in the context of sterile neutrinos produced via neutrino mixing, these observations provide the leading constraints in the mass range 10-12 keV, improving upon previous constraints in this range by a factor ∼2. We also compare our results to Monte Carlo simulations, showing that the fluctuations in our derived limit are not dominated by systematic effects. An updated model of the instrumental background, which is currently under development, will improve NuSTAR's sensitivity to anomalous x-ray lines, particularly for energies 3-5 keV.

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