One of possible explanations of a faint narrow emission line at 3.5 keV reported in our Galaxy, Andromeda galaxy and a number of galaxy clusters is the dark matter made of 7 keV sterile neutrinos. Another signature of such sterile neutrino dark matter could be fewer ionizing sources in the early Universe (compared to the standard "cold dark matter" (CDM) scenario), which should affect the reionization of the Universe. By using a semi-analytical model of reionization, we compare the model predictions for CDM and two different models of 7 keV sterile neutrino dark matter (consistent with the 3.5 keV line interpretation as decaying dark matter line) with available observations of epoch of reionization (including the final measurements of electron scattering optical depth made by Planck observatory). We found that both CDM and 7 keV sterile neutrino dark matter well describe the data. The overall fit quality for sterile neutrino dark matter is slightly (with $\Delta \chi^2 \simeq 2-3$) better than for CDM, although it is not possible to make a robust distinction between these models on the basis of the given observations.

The nearby Perseus galaxy cluster is a key target for indirect detection searches for decaying dark matter. We use the C-EAGLE simulations of galaxy clusters to predict the flux, width, and shape of a dark matter decay line, paying particular attention to the unexplained 3.55. keV line detected in the spectra of some galaxies and clusters, and the upcoming XRISM X-ray observatory mission. We show that the line width in C-EAGLE clusters similar to Perseus is typically [600-800] km s(-1), and therefore narrower than the amplitude of the velocity dispersion of galaxies in the cluster. Halos that are significantly disturbed can, however, exhibit galaxy velocity dispersions higher than 1000. km s(-1), and in this case will show a large difference between the line profiles of on- and off-center observations. We show that the line profile is likely to be slightly asymmetric, but still well approximated by a Gaussian at the 10% level, and that the halo asymmetry can lead to fluxes that vary by a factor of two. In summary, we predict that, if the previously reported 3.55. keV line detections do originate from dark matter decay, the XRISM mission will detect a line with a roughly Gaussian profile at a rest-frame energy of 3.55. keV, with a width > 600 km s(-1). and flux approximately in the range [4-9] x 10(-8) counts s(-1) cm(-2).

Observations of the redshifted 21-cm signal (in absorption or emission) allow us to peek into the epoch of "dark ages" and the onset of reionization. These data can provide a novel way to learn about the nature of dark matter, in particular about the formation of small size dark matter halos. However, the connection between the formation of structures and 21-cm signal requires knowledge of stellar to total mass relation, escape fraction of UV photons, and other parameters that describe star formation and radiation at early times. This baryonic physics depends on the properties of dark matter and in particular in warm-dark-matter (WDM) models, star formation may follow a completely different scenario, as compared to the cold-dark-matter case. We use the recent measurements by the EDGES [J. D. Bowman, A. E. E. Rogers, R. A. Monsalve, T. J. Mozdzen, and N. Mahesh, An absorption profile centred at 78 megahertz in thesky-averaged spectrum,Nature (London) 555, 67 (2018).] to demonstrate that when taking the above considerations into account, the robust WDM bounds are in fact weaker than those given by the Lyman-$\alpha$ forest method and other structure formation bounds. In particular, we show that resonantly produced 7 keV sterile neutrino dark matter model is consistent with these data. However, a holistic approach to modelling of the WDM universe holds great potential and may in the future make 21-cm data our main tool to learn about dark matter clustering properties.

One of the future instruments to resolve the origin of the unidentified 3.5 keV emission line is the Micro-X sounding rocket telescope. According to the estimate made in 2015, Micro-X will be able to detect on average about 18.2 photons from the 3.5 keV line during its 300-second-long planned observation. However, this estimate is based on the extrapolation of the 3.5 keV line signal from the innermost Galactic Centre (GC) region available in 2015. With newly available reports on the 3.5 keV line emission in five off-centre regions, we found that similar Micro-X payload will result in 3.4-4.3 counts on average, depending on the dark matter distribution. Therefore, we show that the 3.5 keV line is unlikely to be detected with a single Micro-X launch using an original Micro-X payload. Increasing its field-of-view from 20$^\circ$ to 33$^\circ$ and its repointing out of GC (to avoid the brightest X-ray point source on the sky, Sco X-1) will increase the expected number of counts from 3.5 keV line to 7.5-7.9, which corresponds to its expected marginal ($\sim 2\sigma$) detection within a single Micro-X observation.

We report a detection of 3.5 keV line in the Milky Way in 5 regions offset from the Galactic Center by distances from 10' to 35 degrees. We build an angular profile of this line and compare it with profiles of several astrophysical lines detected in the same observations. We compare our results with other detections and bounds previously obtained using observations of the Milky Way.

Some of recent detections of a narrow emission line at similar to 3.5 keV have been accompanied by subsequent non-detections in the same sources, raising discussion about the actual level of systematic errors. In this paper, we study the systematics caused by an imperfect knowledge of a continuum model. Our simple theoretical estimate and detailed modeling of simulated spectra allow us to calculate the value of this "continuum-induced" systematics for the first time. We show that, for some objects such as the M31 central part or the Draco dwarf spheroidal galaxy, the obtained level of systematics within a well-defined continuum model allows one to fully reconcile the controversial results claimed previously by different groups of authors. To minimize the effect of "continuum-induced" systematics, we show that one should reasonably decrease the size of the spectral bin and increase the modeled energy range.

We present results of a search for the 3.5 keV emission line in our recent very long (similar to 1.4 Ms) XMM-Newton observation of the Draco dwarf spheroidal galaxy. The astrophysical X-ray emission from such dark matter-dominated galaxies is faint, thus they provide a test for the dark matter origin of the 3.5 keV line previously detected in other massive, but X-ray bright objects, such as galaxies and galaxy clusters. We do not detect a statistically significant emission line from Draco; this constrains the lifetime of a decaying dark matter particle to tau > (7-9) x 10(27) s at 95 per cent CL (combining all three XMM-Newton cameras; the interval corresponds to the uncertainty of the dark matter column density in the direction of Draco). The PN camera, which has the highest sensitivity of the three, does show a positive spectral residual (above the carefully modelled continuum) at E = 3.54 +/- A 0.06 keV with a 2.3 sigma significance. The two MOS cameras show less-significant or no positive deviations, consistently within 1 sigma with PN. Our Draco limit on tau is consistent with previous detections in the stacked galaxy clusters, M31 and the Galactic Centre within their 1 - 2 sigma uncertainties, but is inconsistent with the high signal from the core of the Perseus cluster (which has itself been inconsistent with the rest of the detections). We conclude that this Draco observation does not exclude the dark matter interpretation of the 3.5 keV line in those objects.

The detection of an unidentified emission line in X-ray spectra of cosmic objects would be a 'smoking gun' signature for particle physics beyond the Standard Model. More than a decade of its extensive searches results in several narrow faint emission lines reported at 3.5, 8.7, 9.4 and 10.1 keV. The most promising of them is the emission line at ~3.5 keV reported in spectra of several nearby galaxies and galaxy clusters. Here I summarize its up-to-date status, overview its possible interpretations, including an intriguing connection with radiatively decaying dark matter, and outline future directions for its studies.

The recent discovery of the unidentified emission line at 3.5 keV in galaxies and clusters has attracted great interest from the community. As the origin of the line remains uncertain, we study the surface brightness distribution of the line in the Perseus cluster since that information can be used to identify its origin. We examine the flux distribution of the 3.5 keV line in the deep Suzaku observations of the Perseus cluster in detail. The 3.5 keV line is observed in three concentric annuli in the central observations, although the observations of the outskirts of the cluster did not reveal such a signal. We establish that these detections and the upper limits from the non-detections are consistent with a dark matter decay origin. However, absence of positive detection in the outskirts is also consistent with some unknown astrophysical origin of the line in the dense gas of the Perseus core, as well as with a dark matter origin with a steeper dependence on mass than the dark matter decay. We also comment on several recently published analyses of the 3.5 keV line.

Recent reports of a weak unidentified emission line at 3.5 keV found in spectra of several matter-dominated objects may give a clue to resolve the long-standing problem of dark matter. One of the best physically motivated particle candidate able to produce such an extra line is sterile neutrino with the mass of 7 keV. Previous works show that sterile neutrino dark matter with parameters consistent with the new line measurement modestly affects structure formation compared to conventional cold dark matter scenario. In this work, we concentrate for the first time on contribution of the sterile neutrino dark matter able to produce the observed line at 3.5 keV, to the process of reionization. By incorporating dark matter power spectra for 7 keV sterile neutrinos into extended semi-analytical 'bubble' model of reionization we obtain that such sterile neutrino dark matter would produce significantly sharper reionization compared to widely used cold dark matter models, impossible to 'imitate' within the cold dark matter scenario under any reasonable choice of our model parameters, and would have a clear tendency of lowering both the redshift of reionization and the electron scattering optical depth (although the difference is still below the existing model uncertainties). Further dedicated studies of reionization (such as 21 cm measurements or studies of kinetic Sunyaev-Zeldovich effect) will thus be essential for reconstruction of particle candidate responsible the 3.5 keV line.

Previously, [1207.0458] proposed an astrophysical explanation of narrow gamma-ray line-like feature(s) at ~100 GeV from Galactic Center region observed by Fermi/LAT [1205.4700]. The model of [1207.0458] is based on the inverse Compton scattering of external ultra-violet/X-ray radiation by a cold ultra-relativistic electron-positron pulsar wind. We show that the extra broad ~30 MeV component should arise from Comptonization of cosmic microwave background radiation. We estimate the main parameters of this component and show that it can be detectable with MeV telescopes such as CGRO/COMPTEL. The location of CGRO/COMPTEL unidentified source GRO J1823-12 close to excess of 105-120 GeV emission (Reg.1 of [1205.4700]) can be interpreted as an argument in favour of astrophysical model of the narrow feature(s) at ~100 GeV.

The recent discovery of the unidentified emission line at 3.5 keV in galaxies and clusters has attracted great interest from the community. As the origin of the line remains uncertain, we study the surface brightness distribution of the line in the Perseus cluster since that information can be used to identify its origin. We examine the flux distribution of the 3.5 keV line in the deep Suzaku observations of the Perseus cluster in detail. The 3.5 keV line is observed in three concentric annuli in the central observations, although the observations of the outskirts of the cluster did not reveal such a signal. We establish that these detections and the upper limits from the non-detections are consistent with a dark matter decay origin. However, absence of positive detection in the outskirts is also consistent with some unknown astrophysical origin of the line in the dense gas of the Perseus core, as well as with a dark matter origin with a steeper dependence on mass than the dark matter decay. We also comment on several recently published analyses of the 3.5 keV line.

If the unidentified emission line at ~3.55 keV previously found in spectra of nearby galaxies and galaxy clusters is due to radiatively decaying dark matter, one should detect the signal of comparable strength from many cosmic objects of different nature. By studying existing dark matter distributions in galaxy clusters we identified top-19 of them observed by XMM-Newton X-ray cosmic mission, and analyzed the data for the presence of the new line. In 8 of them, we identified > 2 sigma positive line-like residuals with average position 3.52 +/- 0.08 keV in the emitter's frame. Their observed properties are unlikely to be explained by statistical fluctuations or astrophysical emission lines; observed line position in M31 and Galactic Center makes an additional argument against general-type systematics. Being interpreted as decaying dark matter line, the new detections correspond to radiative decay lifetime tau_dm ~ (3.5-6) x 10^27 s consistent with previous detections.

We detect a line at 3.539±0.011  keV in the deep exposure data set of the Galactic center region, observed with the x-ray multi-mirror mission Newton. The dark matter interpretation of the signal observed in the Perseus galaxy cluster, the Andromeda galaxy [A. Boyarsky et al., Phys. Rev. Lett. 113, 251301 (2014)], and in the stacked spectra of galaxy clusters [E. Bulbul et al., Astrophys. J. 789, 13 (2014)], together with nonobservation of the line in blank-sky data, put both lower and upper limits on the possible intensity of the line in the Galactic center data. Our result is consistent with these constraints for a class of Milky Way mass models, presented previously by observers, and would correspond to the radiative decay dark matter lifetime, τDM∼6-8×10(27)  sec. Although it is hard to exclude an astrophysical origin of this line based on the Galactic center data alone, this is an important consistency check of the hypothesis that encourages us to check it with more observational data that are expected by the end of 2015.

Whether the new line at ~3.5 keV, recently detected in different samples of galaxy clusters, Andromeda galaxy and central part of our Galaxy, is due to Potassium emission lines, is now unclear. By using the latest astrophysical atomic emission line database AtomDB v. 3.0.2, we show that the most prospective method to directly check its Potassium origin will be the study of K XIX emission line complex at ~3.7 keV with future X-ray imaging spectrometers such as Soft X-ray spectometer on-board Astro-H mission or microcalorimeter on-board Micro-X sounding rocket experiment. To further reduce the remaining (factor ~3-5) uncertainty of the 3.7/3.5 keV ratio one should perform more precise modeling including removal of significant spatial inhomogeneities, detailed treatment of background components, and further extension of the modeled energy range.

Emission line at the energy ~3.55 keV detected in different galaxies and galaxy clusters has caused a lot of discussion in high-energy astrophysics and particle physics communities. To reveal the origin of the line, we analyzed publicly available observations of MOS cameras from XMM-Newton cosmic observatory - the instrument with the largest sensitivity for narrow faint X-ray lines - previously combined in X-ray sky maps. Because of extremely large timescale needed for detailed analysis, we used the wavelet method instead. Extensive simulations of the central part of Andromeda galaxy are used to check the validity of this method. The resulting list of wavelet detections now contains 235 sky regions. This list will be used in future works for more detailed spectral analysis.

Large observatory for x-ray timing (LOFT) is a concept of a next-generation x-ray telescope considered in the context of the "Cosmic Vision" program of the European Space Agency. The Large Area Detector on board of LOFT will be a collimator-type telescope with an unprecedentedly large collecting area of about 10(5) cm(2) in the energy band between 2 and 100 keV. We demonstrate that LOFT will be a powerful dark matter detector, suitable for the search of the x-ray line emission expected from decays of light dark matter particles in galactic halos. We show that LOFT will have sensitivity for dark matter line search more than an order of magnitude higher than that of all existing x-ray telescopes. In this way, LOFT will be able to provide a new insight into the fundamental problem of the nature of dark matter.

We report a weak line at 3.52±0.02  keV in x-ray spectra of the Andromeda galaxy and the Perseus galaxy cluster observed by the metal-oxide-silicon (MOS) and p-n (PN) CCD cameras of the XMM-Newton telescope. This line is not known as an atomic line in the spectra of galaxies or clusters. It becomes stronger towards the centers of the objects; is stronger for Perseus than for M31; is absent in the spectrum of a deep "blank sky" data set. Although for each object it is hard to exclude that the feature is due to an instrumental effect or an atomic line, it is consistent with the behavior of a dark matter decay line. Future (non-)detections of this line in multiple objects may help to reveal its nature.

We revisit the X-ray spectrum of the central 14' of the Andromeda galaxy, discussed in our previous work [1402.4119]. Recently in [1408.1699] it was claimed that if one limits the analysis of the data to the interval 3-4 keV, the significance of the detection of the line at 3.53 keV drops below 2 sigma. In this note we show that such a restriction is not justified, as the continuum is well-modeled as a power law up to 8 keV, and parameters of the background model are well constrained over this larger interval of energies. This allows for a detection of the line at 3.53 keV with a statistical significance greater than ~3 sigma and for the identification of several known atomic lines in the energy range 3-4 keV. Limiting the analysis to the 3-4 keV interval results in increased uncertainty, thus decreasing the significance of the detection. We also argue that, with the M31 data included, a consistent interpretation of the 3.53 keV line as an atomic line of K XVIII in all studied objects is problematic.