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Register a new userDetection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
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We detect a weak unidentified emission line at E=(3.55-3.57)+/-0.03 keV in a stacked XMM spectrum of 73 galaxy clusters spanning a redshift range 0.01-0.35. MOS and PN observations independently show the presence of the line at consistent energies. When the full sample is divided into three subsamples (Perseus, Centaurus+Ophiuchus+Coma, and all others), the line is significantly detected in all three independent MOS spectra and the PN all others spectrum. It is also detected in the Chandra spectra of Perseus with the flux consistent with XMM (though it is not seen in Virgo). However, it is very weak and located within 50-110eV of several known faint lines, and so is subject to significant modeling uncertainties. On the origin of this line, we argue that there should be no atomic transitions in thermal plasma at this energy. An intriguing possibility is the decay of sterile neutrino, a long-sought dark matter particle candidate. Assuming that all dark matter is in sterile neutrinos with m_s=2E=7.1 keV, our detection in the full sample corresponds to a neutrino decay mixing angle sin^2(2theta)=7e-11, below the previous upper limits. However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model. This appears to be because of an anomalously bright line at E=3.62 keV in Perseus, possibly an Ar XVII dielectronic recombination line, although its flux would be 30 times the expected value and physically difficult to understand. In principle, such an anomaly might explain our line detection in other subsamples as well, though it would stretch the line energy uncertainties. Another alternative is the above anomaly in the Ar line combined with the nearby 3.51 keV K line also exceeding expectation by factor 10-20. Confirmation with Chandra and Suzaku, and eventually Astro-H, are required to determine the nature of this new line.(ABRIDGED)
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We report the first statistical detection of X-ray emission from cosmic web filaments in ROSAT data. We selected 15,165 filaments at 0.2<z<0.6 ranging from 30 Mpc to 100 Mpc in length, identified in the Sloan Digital Sky Survey (SDSS) survey. We stacked the X-ray count-rate maps from ROSAT around the filaments, excluding resolved galaxy groups and clusters above the mass of ~3 * 10^13 Msun as well as the detected X-ray point sources from the ROSAT, Chandra, and XMM-Newton observations. The stacked signal results in the detection of the X-ray emission from the cosmic filaments at a significance of 4.2 sigma in the energy band of 0.56-1.21 keV. The signal is interpreted, assuming the Astrophysical Plasma Emission Code (APEC) model, as an emission from the hot gas in the filament-core regions with an average gas temperature of 0.9(+1.0-0.6) keV and a gas overdensity of ~30 at the center of the filaments. Furthermore, we show that stacking the SRG/eROSITA data for ~2,000 filaments only would lead to a ~5 sigma detection of their X-ray signal, even with an average gas temperature as low as ~0.3 keV.
We perform a detailed study of the stacked Suzaku observations of 47 galaxy clusters, spanning a redshift range of 0.01-0.45, to search for the unidentified 3.5 keV line. This sample provides an independent test for the previously detected line. We detect only a 2sigma-significant spectral feature at 3.5 keV in the spectrum of the full sample. When the sample is divided into two subsamples (cool-core and non-cool core clusters), cool-core subsample shows no statistically significant positive residuals at the line energy. A very weak (2sigma-confidence) spectral feature at 3.5 keV is permitted by the data from the non-cool core clusters sample. The upper limit on a neutrino decay mixing angle from the full Suzaku sample is consistent with the previous detections in the stacked XMM-Newton sample of galaxy clusters (which had a higher statistical sensitivity to faint lines), M31, and Galactic Center at a 90% confidence level. However, the constraint from the present sample, which does not include the Perseus cluster, is in tension with previously reported line flux observed in the core of the Perseus cluster with XMM-Newton and Suzaku.
We present a study of X-ray AGN overdensities in 16 Abell clusters, within the redshift range 0.073<z<0.279, in order to investigate the effect of the hot inter-cluster environment on the triggering of the AGN phenomenon. The X-ray AGN overdensities, with respect to the field expectations, were estimated for sources with L_x>= 10^{42} erg s^{-1} (at the redshift of the clusters) and within an area of 1 h^{-1}_{72} Mpc radius (excluding the core). To investigate the presence or not of a true enhancement of luminous X-ray AGN in the cluster area, we also derived the corresponding optical galaxy overdensities, using a suitable range of $r$-band magnitudes. We always find the latter to be significantly higher (and only in two cases roughly equal) with respect to the corresponding X-ray overdensities. Over the whole cluster sample, the mean X-ray point-source overdensity is a factor of ~4 less than that corresponding to bright optical galaxies, a difference which is significant at a >0.995 level, as indicated by an appropriate t-student test. We conclude that the triggering of luminous X-ray AGN in rich clusters is strongly suppressed. Furthermore, searching for optical SDSS counterparts of all the X-ray sources, associated with our clusters, we found that about half appear to be background QSOs, while others are background and foreground AGN or stars. The true overdensity of X-ray point sources, associated to the clusters, is therefore even smaller than what our statistical approach revealed.
The presence of luminous hot X-ray coronae in the dark matter halos of massive spiral galaxies is a basic prediction of galaxy formation models. However, observational evidence for such coronae is very scarce, with the first few examples having only been detected recently. In this paper, we study the large-scale diffuse X-ray emission associated with the massive spiral galaxy NGC266. Using ROSAT and Chandra X-ray observations we argue that the diffuse emission extends to at least ~70 kpc, whereas the bulk of the stellar light is confined to within ~25 kpc. Based on X-ray hardness ratios, we find that most of the diffuse emission is released at energies <1.2 keV, which indicates that this emission originates from hot X-ray gas. Adopting a realistic gas temperature and metallicity, we derive that in the (0.05-0.15)r_200 region (where r_200 is the virial radius) the bolometric X-ray luminosity of the hot gas is (4.3 +/- 0.8) x 10^40 erg/s and the gas mass is (9.1 +/- 0.9) x 10^9 M_sun. These values are comparable to those observed for the two other well-studied X-ray coronae in spiral galaxies, suggesting that the physical properties of such coronae are similar. This detection offers an excellent opportunity for comparison of observations with detailed galaxy formation simulations.
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