Constraining dark matter properties with SPI

Abstract

Using the high-resolution spectrometer SPI on board the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) , we search for a spectral line produced by a dark matter (DM) particle with a mass in the range40keV < M D M < 14M eV , decaying in the DM halo of the Milky Way. To distinguish the DM decay line from numerous instrumental lines found in the SPI background spectrum, we study the dependence of the intensity of the line signal on the offset of the SPI pointing from the direction toward th e Galactic Centre. After a critical analysis of the uncertainties of the DM density profile in the inner Galaxy, we find that the intensity of the DM decay line should decrease by at least a factor of 3 when the offset from the Galactic Centre increases from 0 � to 180 � . We find that such a pronounced variation of the line flux across the sky is not observed for any line, detec ted with a significance higher than 3� in the SPI background spectrum. Possible DM decay origin is not ruled out only for the unidentified spectral lines, having low ( � 3�) significance or coinciding in position with the instrumental ones. In the energy interval from 20 keV to 7 MeV, we derive restrictions on the DM decay line flux, implied by the (non-)detection of the D M decay line. For a particular DM candidate, the sterile neutrino of mass M D M , we derive a bound on the mixing angle.