Decaying dark matter search with NuSTAR deep sky observations

We present the results of the search for decaying dark matter with particle mass in the 6-40 keV range with NuSTAR deep observations of COSMOS and ECDFS empty sky fields. We show that the main contribution to the decaying dark matter signal from the Milky Way galaxy comes through the aperture of the NuSTAR detector, rather than through the focusing optics. The high sensitivity of the NuSTAR detector, combined with the large aperture and large exposure times of the two observation fields, allows us to improve previously existing constraints on the dark matter decay time by up to an order of magnitude in the mass range 10-30 keV. In the particular case of the nu MSM sterile neutrino dark matter, our constraints impose an upper bound m < 20 keV on the dark matter particle mass. We report detection of four unidentified spectral lines (including the line at 3.51 keV) in our data set. These line detections are either due to the systematic effects (uncertainties of calibrations of the NuSTAR detectors) or have an astrophysical origin. We discuss different possibilities for testing the nature of the detected lines.