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 $\nu \mathrm{MSM}$ sterile neutrino dark matter, our constraints impose an upper bound $m<20\mathrm{keV}$ 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.

Figure 1

Left: COSMOS (red and black points for NuSTAR A and B modules) and ECDFS (blue and green points correspondingly) fields spectra in 3–70 keV energy band with the best-fit considered background model (see text for the details). Right: $3\sigma$ upper limits on a narrow gaussian line flux added on top of the background model. The results for “all” and “no-Sun” data sets are shown with solid red and dashed blue lines correspondingly. The solid green line shows the upper limits estimated from simulations [46], see text for the details.

Figure 2

The ratio of the aperture and the focused parts of the dark matter signal as a function of energy.

Figure 3

Constraints on sterile DM mass $m_{\mathrm{DM}}$ and mixing angle ${\sin }^2(2{\theta }_{\mathrm{DM}})$ from NuSTAR observations of COSMOS and ECDFS fields. Light and dark blue shadings show the results from the “all” and “no-Sun” data sets. Darker grey shows previously derived constraints from x-ray observations [11, 12, 13, 14, 21, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 36, 37]. Black lines delimit the allowed range of parameters of $\nu \mathrm{MSM}$ model. Dashed red curve shows the sensitivity reach of Athena/XIFU observations of dwarf spheroidal galaxies [46]. Data point marks the estimate of the dark matter sterile neutrino mixing angle for the dark matter interpretation of the 3.5 keV line in galaxy clusters and in M31. Upward arrows mark the masses of dark matter particles which would produce the $\gtrsim 3\sigma$ lines found in the NuSTAR spectra of COSMOS and ECDFS fields.