NuSTAR Tests of Sterile-Neutrino Dark Matter: New Galactic Bulge Observations and Combined Impact

TL;DR

This study uses NuSTAR observations near the Galactic bulge to search for sterile-neutrino dark matter decay signals, setting new constraints on their properties in the 10-12 keV mass range.

Contribution

First to analyze dedicated NuSTAR data from the Galactic bulge for sterile-neutrino decay signals, providing the most stringent constraints in the 10-12 keV mass range.

Findings

No evidence of x-ray lines from sterile-neutrino decay in 5-20 keV range.

Constraints in the 10-12 keV mass range are improved by a factor of about 2.

Fluctuations in limits are not dominated by systematic effects.

Abstract

We analyze two dedicated NuSTAR observations with exposure ${\sim}190$ ks located ${\sim}10^\circ$ 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.