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

Aims. We improve the earlier restrictions on parameters of the warm dark matter (DM) in the form of a sterile neutrinos. Methods. The results were obtained from not 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. Results. The new Milky way data improve on ( by as much as the order of magnitude at masses similar to 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 M 31. 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.