We propose a strategy for how to look for dark matter particles possessing a radiative decay channel and derive constraints on their parameters from observations of x rays from our own Galaxy and its dwarf satellites. When applied to sterile neutrinos in the keV mass range this approach gives a significant improvement to restrictions on neutrino parameters compared with previous works. DOI: 10.1103/PhysRevLett.97.261302 PACS numbers: 95.35.+d, 14.60.Pq, 95.85.Nv Introduction.-It was noticed long ago that a sterile neutrino with mass in the keV range appears to be a viable dark matter (DM) candidate [1]. Moreover, being warm DM, a sterile neutrino eases the tension between observations and predictions of the cold DM model on small scales. The interest in this scenario has been revived since the discovery of neutrino oscillations (see, e.g., [2] for a review). Indeed, one of the simplest ways to explain these data is to add to the standard model (SM) several gauge singlet fermions-right-handed, or sterile, neutrinos. It has been demonstrated recently [3] that a simple extension of the SM by three singlet fermions with masses smaller than the electroweak scale, dubbed the MSM in [3], allows one to describe all confirmed data on neutrino oscillations, provides a DM particle candidate in the form of a sterile neutrino, and allows one to explain the baryon asymmetry of the Universe. The simplicity of the model, the similarity of its quark and lepton right-handed sectors, together with a considerable number of other phenomena it can simultaneously describe, forces us to take this model seriously and thus provides additional motivation for the study of keV mass range sterile neutrinos as a DM candidate.The sterile neutrino has a radiative decay channel, emitting a photon with energy E m s =2 (m s being the mass of the sterile neutrino). Parametrically, the decay width is proportional to m 5 s sin 2 2 [4], where is the mixing angle between active and sterile neutrino.If such a neutrino is a main ingredient of the DM, it is potentially detectable in various x-ray observations. The most obvious candidates are diffuse extragalactic x-ray background (XRB) [5][6][7][8], clusters of galaxies [6,9,10], and galaxies [6], including our own.The aim of the present Letter is to discuss the best strategy to search for a DM sterile neutrino and to derive the constraints on its properties. Although we concentrate on the sterile neutrino, the constraints we get can be applied to any DM candidate with a radiative two-body decay channel in the keV range. We analyze various types of astrophysical objects and show that the strongest constraints on sterile neutrino come from neutrino decays in the Milky Way halo and, in particular, in the halo dwarf

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 range 40 keV < MDM < 14 MeV, 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 the 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, detected with a significance higher than 3σ in the SPI background spectrum. Possible DM decay origin is not ruled out only for the unidentified spectral lines, having low (∼3σ) 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 DM decay line. For a particular DM candidate, the sterile neutrino of mass MDM, we derive a bound on the mixing angle.

Several recent works have reported the detection of an unidentified X-ray line at 3.55 keV, which could possibly be attributed to the decay of dark matter (DM) particles in the halos of galaxy clusters and in the M31 galaxy. We analyze all publicly-available XMM-Newton data of dwarf spheroidal galaxies to test the possible DM origin of the line. Dwarf spheroidal galaxies have high mass-to-light ratios and their interstellar medium is not a source of diffuse X-ray emission, thus they are expected to provide the cleanest DM decay line signal. Our analysis shows no evidence for the presence of the line in the stacked spectra of the dwarf galaxies. It excludes the sterile neutrino DM decay origin of the 3.5 keV line reported by Bulbul et al. (2014) at the level of 4.1σ under standard assumptions about the Galactic DM column density in the direction of selected dwarf galaxies and at the level of 3.2σ assuming minimal Galactic DM column density. Our analysis is still consistent with the estimate of sterile neutrino DM parameters by , because of its larger uncertainty. However, the central value of their estimate of the mixing angle is inconsistent with our dwarf spheroidals data at 3.4σ (2.5σ) level assuming the mean (minimal) Galactic DM column density. As a by-product of our analysis, we provide updated upper limits to the mixing angle of sterile neutrino DM in the mass range between 2 and 20 keV.

We find restrictions on the mass and mixing angle of the dark matter sterile neutrinos using x-ray observations of Coma and Virgo galaxy clusters with XMM-Newton satellite. In the absence of clearly detectable line, we present detailed analysis of various methods of putting restrictions on mass and mixing angle of sterile neutrino. Our analysis provides significant improvement over our previous results, coming from XRB background measurements. We also discuss restrictions from Virgo cluster by other authors and compare our results with them.