Constraints on the parameters of radiatively decaying dark matter from the dark matter halo of the Milky Way and Ursa Minor

TL;DR

The paper constrains keV-scale sterile-neutrino dark matter that decays radiatively by searching for the resulting X-ray line in Milky Way halo and Ursa Minor observations with XMM-Newton PN. It models the Milky Way DM halo with an NFW profile and uses blank-sky data to convert non-detections into limits on the decay width through the line flux $F_{ m DM}$ and energy $E_ extgamma = M_s/2$. The authors find up to an order-of-magnitude improvement over previous MW-based bounds around $M_s \approx 3.5$ keV, and Ursa Minor provides competitive limits despite limited exposure, supporting dwarfs as promising DM-decay targets. The work emphasizes that combining multiple DM-dominated environments reduces modeling uncertainties and that the approach generalizes to any DM candidate with a monoenergetic radiative decay channel, $\Gamma \propto \sin^2(2\theta) M_s^5$ and $E_\gamma = M_s/2$.

Abstract

We improve the earlier restrictions on parameters of the dark matter (DM) in the form of a sterile neutrino. The results were obtained from non-observing the DM decay line in the X-ray spectrum of the Milky Way (using the recent XMM-Newton PN blank sky data). We also present a similar constraint coming from the recent XMM-Newton observation of Ursa Minor -- dark, X-ray quiet dwarf spheroidal galaxy. The new Milky way data improve on (by as much as the order of magnitude at masses ~3.5 keV) existing constraints. Although the observation of Ursa Minor has relatively poor statistics, the constraints are comparable to those recently obtained using observations of the Large Magellanic Cloud or M31. This confirms a recent proposal that dwarf satellites of the MW are very interesting candidates for the DM search and dedicated studies should be made to this purpose.

Figures (8)

• Figure 1: Upper panel: The blank sky data of Nevalainen:05 after subtraction of the closed filter data. The black crosses show the data points, used for modeling the Galactic emission and CXB. The blue crosses show the data points excluded from the fit. The best-fit model is shown with the red line. The plotted spectrum is binned by a minimum of 20000 counts per channel, different from that used in the actual fit (see text). The error bars include the 5% systematic uncertainty used in the analysis. Lower panel: The ratio of the data-to-model values in each channel used in the fit.
• Figure 2: Exclusion plot based on the blank sky observations. Bins $2.9-3.1\:\mathrm{keV}\xspace$ and $11.6-12.6\:\mathrm{keV}\xspace$ are excluded.
• Figure 3: Dependence of the results on closed filter normalization. Red (solid) and green (long-dashed) lines are the same as in Fig. \ref{['fig:mw']}.
• Figure 4: Hard band light curve for the UMi observation 0301690401. The crosses show the PN $>$10 keV band rate of the full FOV in 1ks bins. The dashed line shows the average, when excluding first 2 time bins. The dotted line shows the corresponding quiescent value in the co-added blank sky data Nevalainen:05.
• Figure 5: Flux from UMi (obs. ID 0301690401). Energy bins have the width of twice the spectral resolution. Shown are the 1, 2, and 3$\sigma$ errors. One can see that, above 2 keV, flux in most energy bins is zero within $1\sigma$ limits (blue crosses) and for the rest it is zero within $2\sigma$ limits (green crosses). Similarly, below 2 keV black, cyan and yellow crosses represent 1,2, and 3$\sigma$ error correspondingly. The solid black line represents the 3$\sigma$ upper bound on total flux in a given energy bin, which we use to put the limit on DM parameters.