Sterile neutrinos with the mass in the keV range are interesting warm dark matter (WDM) candidates. The restrictions on their parameters (mass and mixing angle) obtained by current Xray missions (XMM-Newton or Chandra) can only be improved by less than an order of magnitude in the near future. Therefore the new strategy of search is needed. We compare the sensitivities of existing and planned X-ray missions for the detection of WDM particles with the mass ∼ 1−20 keV. We show that existing technology allows an improvement in sensitivity by a factor of 100. Namely, two different designs can achieve such an improvement: [A] a spectrometer with the high spectral resolving power of 0.1%, wide (steradian) field of view, with small effective area of about cm 2 (which can be achieved without focusing optics) or [B] the same type of spectrometer with a smaller (degree) field of view but with a much larger effective area of 10 3 cm 2 (achieved with the help of focusing optics). To illustrate the use of the "type A" design we present the bounds on parameters of the sterile neutrino obtained from analysis of the data taken by an X-ray microcalorimeter. In spite of the very short exposure time (100 sec) the derived bound is comparable to the one found from long XMM-Newton observation.
I. STERILE NEUTRINO AS WDM CANDIDATE.During the last year a number of works appeared, devoted to search for the decay signal of a DM candidate -sterile neutrino -in the X-ray spectra of astrophysical objects [1,2,3,4,5,6,7,8,9]. Indeed, it was noticed long ago [10] that a right-handed neutrino with its mass in the keV range presents a viable warm dark matter (WDM) candidate. Such a particle would possess a specific radiative decay channel and therefore one can search for its decay line in the X-ray spectra of astrophysical objects [11,12].The recent spark of interest in the search for sterile neutrino DM has several reasons. First, for the direct search of a DM particle, a particle physics model is needed. Most of the particle physics candidates (axion, supersymmetric particles, etc.) would constitute cold dark matter (CDM). CDM models have several difficulties which could be resolved by a warm DM with the particle mass in the keV range. In particular, WDM can ease the problem of the dark halo structures in comparison with the CDM scenario [13,14,15]. Second, as the Standard Model of particle physics (SM) does not contain a DM candidate, most of the extensions of the SM ( like, for example, supersymmetry) require to assume the existence of many new particles and/or validity of new fundamental principles. Such extensions are not based on any available experimental data, but on theoretical arguments only. From this point of view, the extension of the SM with several right-handed neutrinos (i) is, maybe, a minimal extension of the SM one can imagine; (ii) is based on experimental data; (iii) provides naturally a warm DM candidate.Indeed, the existence of right-handed neutrino particles would provide the most natural explanation of neutrino oscill...
