We use NuSTAR observations of the Galactic Center to search for X-ray lines from the radiative decay of sterile neutrino dark matter. Finding no evidence of unknown lines, we set limits on the sterile neutrino mass and mixing angle. In most of the mass range 10-50 keV, these are now the strongest limits, at some masses improving upon previous limits by a factor of ∼ 10. In the νMSM framework, where additional constraints from dark matter production and structure formation apply, the allowed parameter space is reduced by more than half. Future NuSTAR observations may be able to cover much of the remaining parameter space. P r e v io u s X -r a y c o n s t r a in t s M W s a t e ll it e c o u n t s a n d p h a s e s p a c e c o n s t r a in t sNuSTAR GC 2016 FIG. 1. Simplified overview of constraints on νMSM sterile neutrino dark matter in the plane of mass and mixing angle; details are described in Sec. IV, and the experimental constraints included are listed in Fig. 8. For parameters between the gray regions, the observed dark matter abundance can be produced through resonant production in the νMSM. Most of this region is ruled out by constraints from structure formation (blue) or astrophysical X-ray observations (green). Our new constraint (red line and hatched region) is obtained from NuSTAR observations of the GC, and rules out about half of the previously allowed parameter space (white region).

Weakly interacting massive particles (WIMPs) have long reigned as one of the leading classes of dark matter candidates. The observed dark matter abundance can be naturally obtained by freezeout of weakscale dark matter annihilations in the early Universe. This "thermal WIMP" scenario makes direct predictions for the total annihilation cross section that can be tested in present-day experiments. While the dark matter mass constraint can be as high as m χ ≳ 100 GeV for particular annihilation channels, the constraint on the total cross section has not been determined. We construct the first model-independent limit on the WIMP total annihilation cross section, showing that allowed combinations of the annihilationchannel branching ratios considerably weaken the sensitivity. For thermal WIMPs with s-wave 2 → 2 annihilation to visible final states, we find the dark matter mass is only known to be m χ ≳ 20 GeV. This is the strongest largely model-independent lower limit on the mass of thermal-relic WIMPs; together with the upper limit on the mass from the unitarity bound (m χ ≲ 100 TeV), it defines what we call the "WIMP window." To probe the remaining mass range, we outline ways forward.

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]

Dark matter capture and annihilation in the Sun can produce detectable high-energy neutrinos, providing a probe of the dark matter-proton scattering cross section. We consider the case when annihilation proceeds via long-lived dark mediators, which allows gamma rays to escape the Sun and reduces the attenuation of neutrinos. For gamma rays, there are exciting new opportunities, due to detailed measurements of GeV solar gamma rays with Fermi, and unprecedented sensitivities in the TeV range with HAWC and LHAASO. For neutrinos, the enhanced flux, particularly at higher energies (∼TeV), allows a more sensitive dark matter search with IceCube and KM3NeT. We show that these search channels can be extremely powerful, potentially improving sensitivity to the dark matter spin-dependent scattering cross section by several orders of magnitude relative to present searches for high-energy solar neutrinos, as well as direct detection experiments.