Deep Search for Decaying Dark Matter with XMM-Newton Blank-Sky Observations

Sterile neutrinos with masses in the keV range are well-motivated extensions to the Standard Model that could explain the observed neutrino masses while also making up the dark matter (DM) of the universe. If sterile neutrinos are DM then they may slowly decay into active neutrinos and photons, giving rise to the possibility of their detection through narrow spectral features in astrophysical x-ray data sets. In this Letter, we perform the most sensitive search to date for this and other decaying DM scenarios across the mass range from 5 to 16 keV using archival XMM-Newton data. We reduce 547 Ms of data from both the MOS and PN instruments using observations taken across the full sky and then use this data to search for evidence of DM decay in the ambient halo of the Milky Way. We determine the instrumental and astrophysical baselines with data taken far away from the Galactic Center, and use Gaussian process modeling to capture additional continuum background contributions. No evidence is found for unassociated x-ray lines, leading us to produce the strongest constraints to date on decaying DM in this mass range.

Figure 1

Our fiducial $D$ factor, which is proportional to the expected DM signal flux. Values are given in all 30 annuli, which are 6° wide in angular distance from the GC (with $|b|>2°$), and we define a signal and background ROI as shown. In each ring, we compute the $D$ factor of all MOS or PN exposures, weighted according to the observation time and field of view. The horizontal line indicates $D_{\mathrm{bkg}}$, the mean $D$ factor in the background ROI.

Figure 2

The background-subtracted MOS data for the innermost annulus, downbinned by a factor of 4 for presentation purposes. The indicated best-fit null and signal models, for a 3.5 keV UXL, are constructed using the GP modeling described in the text.

Figure 3

Upper panel: the power-constrained 95% upper limit on the DM lifetime from this work, presented in the context of the sterile-neutrino mixing angle ${\sin }^2(2\theta )$, as a function of the DM mass $m_{\chi }$. The dark grey regions correspond to theoretical bounds for DM underproduction in the $\nu \mathrm{MSM}$ or bounds from previous x-ray searches (1)–(5); see text for details. Lower panel: the associated sign-weighted significance for the UXL. Vertical grey regions denote background lines and are at least partially masked. Green and gold regions indicate $1/2\sigma$ expectations under the null hypothesis. These results are shown in the context of more general DM models as constraints on the DM lifetime in the Supplemental Material [42], Fig. S6.