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eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission which is scheduled for launch in 2013. eROSITA will perform an all-sky survey lasting four years, followed by a phase of three years for pointed observations. eROSITA consists of seven identical Mirror Modules, each equipped with 54 Wolter-I shells with an outer diameter of 360 mm. This would provide an effective area of ~1500 cm2at 1.5 keV and an on axis PSF HEW of 15 arcsec resulting in an effective angular resolution of 28 arcsec averaged over the field of view. In the focus of each mirror module, a fast frame-store pn-CCD provides a field of view of 1°in diameter. In this paper we report on the instrument development and its status only.

Galaxy clusters are ideal tracers of the large-scale structure and evolution of the universe. They are thus good probes for the matter content of the universe, the existence of dark matter, and for the statistics of the large-scale structure of the matter distribution. X-ray observations provide a very effective tool to characterize individual galaxy clusters as well as the cluster population. With the detailed analysis of X-ray observations of galaxy clusters the matter composition of clusters is obtained which can be taken as representative of the matter composition of the universe. Based on galaxy cluster surveys in X-rays a census of the galaxy cluster population and statistical measures of the spatial distribution of clusters is obtained. Comparison of the results with predictions from cosmological models yields interesting cosmological model constraints and in particular favours a low density universe.

We describe the ROSATAll-Sky Survey-Center for Astrophysics Loose Systems (RASSCALS), the largest X-ray and optical survey of low-mass galaxy groups to date. We draw 260 groups from the combined Center for Astrophysics and Southern Sky Redshift Surveys, covering one-quarter of the sky to a limiting Zwicky magnitude of mz= 15.5. We detect 61 groups (23%) as extended X-ray sources. The statistical completeness of the sample allows us to make the first measurement of the X-ray selection function of groups, along with a clean determination of their fundamental scaling laws. We find robust evidence of similarity breaking in the relationship between the X-ray luminosity and velocity dispersion. Groups with ?p< 340 km s-1are overluminous by several orders of magnitude compared to the familiar LX?4law for higher velocity dispersion systems. An understanding of this break depends on the detailed structure of groups with small velocity dispersions ?p< 150 km s-1. After accounting for selection effects, we conclude that only 40% of the optical groups are extended X-ray sources. The remaining 60% are either accidental superpositions or systems devoid of X-ray emitting gas. Combining our results with group statistics from N-body simulations, we find that the fraction of real, bound systems in our objectively selected optical catalog is between 40%-80%. The X-ray detections have a median membership of nine galaxies, a median recession velocity cz= 7250 km s-1, a median projected velocity dispersion ?p= 400 km s-1, and a median X-ray luminosity LX= 3 × 1042hergs s-1, where the Hubble constant is H0= 100 h100km s-1Mpc-1. We include a catalog of these properties, or the appropriate upper limits, for all 260 groups.

The results obtained from ASCA observations of the cluster of galaxies Abell 3627 are presented. This cluster, located behind the Milky Way, was recently found to be a nearby, X-ray bright, and very rich cluster. Pointed observations onto the central region of the cluster gave a gas temperature of ∼ 7 keV and a metallicity of about 0.2 solar. An offset pointing to a substructure elongated to the south-east of the cluster center gave a significantly lower temperature of ∼ 5 keV. The 2–10 keV luminosity within a radius of 40′ (1.1 Mpc) is estimated to be 3.7 × 1044erg s−1. The X-ray data imply a cluster mass of about 4× 1014M⊙ within 40′.

X-ray emission from the distant lensing cluster Cl 2236-04 at z= 0.552 was discovered by ASCAand ROSAT/HRI observations. If the spherical symmetric mass distribution model of the cluster is assumed, the lensing estimate of the cluster mass is a factor of 2 higher than that obtained from X-ray observations as reported for many distant clusters. However, the elliptical and clumpy lens model proposed by Kneib and coworkers is surprisingly consistent with the X-ray observations, assuming that the X-ray-emitting hot gas is isothermal and in a hydrostatic equilibrium state. The existence of the cooling flow in the central region of the cluster is indicated by the short central cooling time and the excess flux detected by ROSAT/HRI compared to the ASCAflux. However, it is shown that, even if the Cl 2236-04 has a cooling flow in the central region, the temperature measured by ASCA, which is the mean emission-weighted cluster temperature in this case, should not be different from the virial temperature of the cluster. Therefore, we conclude that the effects of the clumpiness and nonzero ellipticity in the mass distribution of the cluster are essential to explain the observed feature of the giant luminous arc, and in this cluster there is no discrepancy between strong lensing and X-ray estimation of the cluster mass.

We present results of the X-ray analyses of the nearby cluster of galaxies Abell 2399 ($z$= 0.058) using the XMM–Newton and Suzaku satellites. This cluster is classified as a low surface brightness (LSB) cluster. It has a bimodal structure in X-rays, and the X-ray-emission peaks are significantly offset from the peaks in gas temperature. By de-projecting the annular spectra, the temperature and electron density in the central r< 0.1 r200region are measured to be 3.6 keV and 1.4 × 10−3cm−3, respectively. This yields a very high gas entropy, ∼300 keV cm2, in the central region, comparable to the values observed in other LSB clusters. The scaled entropy of Abell 2399 is the highest among the REXCESS cluster sample. The spatial distribution of the member galaxies exhibits multiple substructures, the locations of which are significantly different from those in the X-ray image. We discovered a clear discontinuity in the X-ray brightness and temperature distributions in the western gas clump, which provides evidence of the presence of a cold front. Therefore, our results strongly suggest that A2399 has experienced a merger and that the high central entropy originates from the merger activity.

We present analysis results for a nearby galaxy cluster Abell 1631 at z= 0.046 using the X-ray observatory Suzaku. This cluster is categorized as a low X-ray surface brightness cluster. To study the dynamical state of the cluster, we conduct four-pointed Suzaku observations and investigate physical properties of the Mpc-scale hot gas associated with the A 1631 cluster for the first time. Unlike relaxed clusters, the X-ray image shows no strong peak at the center and an irregular morphology. We perform spectral analysis and investigate the radial profiles of the gas temperature, density, and entropy out to approximately 1.5 Mpc in the east, north, west, and south directions by combining with the XMM-Newton data archive. The measured gas density in the central region is relatively low (a few ×10−4cm−3) at the given temperature (∼2.9 keV) compared with X-ray-selected clusters. The entropy profile and value within the central region (r< 0.1 r200) are found to be flatter and higher (≳400 keV cm2). The observed bolometric luminosity is approximately three times lower than that expected from the luminosity–temperature relation in previous studies of relaxed clusters. These features are also observed in another low surface brightness cluster, Abell 76. The spatial distributions of galaxies and the hot gas appear to be different. The X-ray luminosity is relatively lower than that expected from the velocity dispersion. A post-merger scenario may explain the observed results.