New constraints on sterile neutrino dark matter from NuSTAR M31 observations

Abstract

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]

Suppressed flavor violation in lepton flavored dark matter from an extra dimension

“…In order to gain further insight into the parameter dependence of the constraints, we show in figure 10 how the X-ray flux from χ τ decays (where we use the median value for the χ τ lifetime from 100,000 trials) depends on m χ and m φ , for the parameter point M e L = 8, M L = 1. We also indicate where the exclusion contour lies, namely where exactly 95% of trials leads to an X-ray flux consistent with the bounds [53,54]. When the parameter M e is increased, the contours in this figure will move further up and to the left, making the excluded region smaller, while varying the parameter M will not have a significant effect on the contours.…”

“…These bounds are worked out in ref. [53] for much lighter DM particles decaying according to the mode χ ν → γν, with the result…”

“…Matching this rate between the model used in refs. [53,54] (with DM mass m ν and lifetime τ ν ) and our model, and taking into account that in our model the decaying χ τ only comprise 1/3 of the DM number density, we can set up a correspondence between the bounds on the DM lifetime in the two models:…”

“…In this plot, we also show how the X-ray constraints impact the parameter space. In particular, in the green shaded and m φ , we show the contours for the ratio of the X-ray flux from χ τ decays to the limit on this flux [53,54], using the median value of χ τ lifetime obtained from 100,000 random trials. We also show in red the contour where exactly 95% of the trials are consistent with the bound.…”

“…This is shown in figure 2. Due to the larger mass splitting and thus less phase space suppression, the χ τ lifetime is much shorter compared to the χ µ lifetime, and bounds on the production of keV-range X-rays [53,54] in this decay place severe constraints on the model, δλ/λ < ∼ O(10 −6 ). Note that this is a significantly stronger constraint than the one imposed by µ → eγ, therefore once the X-ray indirect detection bounds are satisfied, flavor violating processes are automatically safe.…”