NuSTAR tests of sterile-neutrino dark matter: New Galactic bulge observations and combined impact

We analyze two dedicated NuSTAR observations with exposure $\sim 190\mathrm{ks}$ located $\sim 10°$ 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 $\sim 2$. 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.

Figure 1

The combined impact on the $\nu \mathrm{MSM}$ parameter space of previous NuSTAR searches [30, 31, 32, 33] and this work is indicated by the green region. This work provides the leading constraints in the 10–12 keV mass range, as shown in Fig. 5. The tentative $E\simeq 3.5\mathrm{keV}$ signal [34, 35, 36] is indicated by the red point. Constraints from other x-ray instruments [37, 38, 39, 40, 41] are shown for comparison. Uncertainties associated with MW satellite counts [28] and BBN [26, 27] are discussed in Sec. 1.

Figure 2

Sky map of the Galactic bulge region. The base color map shows the 17–60 keV flux measured by INTEGRAL [97], with many x-ray point sources clearly visible. The 0-bounce FOVs for the observations analyzed in this paper are indicated by the dashed red (FPMA) and solid blue (FPMB) “Pac-Man”-shaped curves, and avoid known bright x-ray sources. The solid black contours indicate the predicted GRXE flux using the Galactic stellar mass model from Ref. [98] and the GRXE emissivity model from Ref. [99] (see Sec. 2d). The contour values are symmetric about $b=0°$, decrease as $|b|$ increases, and are evenly spaced in ${\log }_{10}(\mathrm{flux})$ between ${10}^{-12.5}–{10}^{-11}\mathrm{erg}{\mathrm{s}}^{-1}{\mathrm{cm}}^{-2}{\mathrm{deg}}^{-2}$, inclusive.

Figure 3

Data and model spectra for obsID 40410001002, with FPMA (left) and FPMB (right), including contributions from the CXB, instrumental background, and the GRXE. The error bars correspond to $\pm 1\sigma$ statistical uncertainties, and the CXB and GRXE curves incorporate both 0-bounce and 2-bounce emission. We exclude the energy range 20–95 keV as it is dominated by internal detector lines (in previous analyses [31, 33], we have already probed this range well), though we include the energy range 95–110 keV to constrain the internal detector continuum. See Sec. 2d for details.

Figure 4

Same as Fig. 3, but for obsID 40410002002.

Figure 5

Left: Comparison of the limit obtained in this paper to that from several surveys using the 0-bounce technique, including blank sky (green, Ref. [32]), Galactic center (red, Ref. [31]), and M31 fields (blue, Ref. [33]), as well as the tentative signal at $E\simeq 3.5\mathrm{keV}$ (red point, Refs. [34, 35, 36]). With only $\sim 190\mathrm{ks}$, we have achieved comparable constraints to analyses with much deeper exposures [31, 32, 33]. We have achieved the best constraint in 10–12 keV mass range, essential for investigating the remaining $\nu \mathrm{MSM}$ parameter space shown in Fig. 1. Right: The observed 95% upper limit on the DM decay rate $\mathrm{\Gamma }$ obtained in this paper, compared to the expected 68% (green) and 95% (yellow) sensitivity bands from simulations (see Sec. 3b).