Ulrich G. Briel

The European Photon Imaging Camera on XMM-Newton: The MOS cameras

The EPIC focal plane imaging spectrometers on XMM-Newton use CCDs to record the images and spectra of celestial X-ray sources focused by the three X-ray mirrors. There is one camera at the focus of each mirror; two of the cameras contain seven MOS CCDs, while the third uses twelve PN CCDs, dening a circular eld of view of 30 0 diameter in each case. The CCDs were specially developed for EPIC, and combine high quality imaging with spectral resolution close to the Fano limit. A lter wheel carrying three kinds of X-ray transparent light blocking lter, a fully closed, and a fully open position, is tted to each EPIC instrument. The CCDs are cooled passively and are under full closed loop thermal control. A radio-active source is tted for internal calibration. Data are processed on-board to save telemetry by removing cosmic ray tracks, and generating X-ray event les; a variety of dierent instrument modes are available to increase the dynamic range of the instrument and to enable fast timing. The instruments were calibrated using laboratory X-ray beams, and synchrotron generated monochromatic X-ray beams before launch; in-orbit calibration makes use of a variety of celestial X-ray targets. The current calibration is better than 10% over the entire energy range of 0.2 to 10 keV. All three instruments survived launch and are performing nominally in orbit. In particular full eld-of-view coverage is available, all electronic modes work, and the energy resolution is close to pre-launch values. Radiation damage is well within pre-launch predictions and does not yet impact on the energy resolution. The scientic results from EPIC amply full pre-launch expectations.

Properties of the X-ray-brightest Abell-type clusters of galaxies (XBACs) from ROSAT All-Sky Survey data — I. The sample

We present an essentially complete, all-sky, X-ray flux limi ted sample of 242 Abell clusters of galaxies (six of which are double) compiled from ROSAT All-Sky Survey data. Our sample is uncontaminated in the sense that systems featuring prominent X-ray point sources such as AGN or foreground stars have been removed. The sample is limited to high Galactic latitudes (jbj� 20 � ), the nominal redshift range of the ACO catalogue of z � 0:2, and X-ray fluxes above 5:0�10 12 erg cm 2 s 1 in the 0.1 ‐ 2.4 keV band. Due to the X-ray flux limit, our sample consists, at intermediate and high redshifts, ex clusively of very X-ray luminous clusters. Since the latter tend to be also optically rich, th e sample is not affected by the optical selection effects and in particular not by the volume incompleteness known to be present in the Abell and ACO catalogues for richness class 0 and 1 clusters. Our sample is the largest X-ray flux limited sample of galaxy c lusters compiled to date and will allow investigations of unprecedented statistica l quality into the properties and distribution of rich clusters in the local Universe.

Probing turbulence in the Coma galaxy cluster

Spatially-resolved gas pressure maps of the Coma galaxy cluster are obtained from a mosaic of XMM-Newton observations in the scale range between a resolution of 20 kpc and an extent of 2.8 Mpc. A Fourier analysis of the data reveals the presence of a scale-invariant pressure fluctuation spectrum in the range between 40 and 90 kpc and is found to be well described by a projected Kolmogorov/Oboukhov-type turbulence spectrum. Deprojection and integration of the spectrum yields the lower limit of ∼10 percent of the total intracluster medium pressure in turbulent form. The results also provide observational constraints on the viscosity of the gas.

The ROSAT all - sky survey bright source catalogue

In order to ensure the quality of the source catalogue derived from the SASS processing an automatic as well as a visual screening procedure was applied to 1378 survey fields. Most (94%) of the 18,811 sources were confirmed by this screening process. The rest is flagged for various reasons. Broad band images are available for a subset of the flagged sources. Details of the screening process can be found at www.rosat.mpe-garching.mpg.de/survey/rass-bsc/doc.html.

The Representative XMM-Newton Cluster Structure Survey (REXCESS) of an X-ray Luminosity Selected Galaxy Cluster Sample

Context.The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass.

The dynamical state of the Coma cluster with XMM-Newton

We present in this paper a substructure and spectroimaging study of the Coma cluster of galaxies based on XMM- Newton data. XMM-Newton performed a mosaic of observations of Coma to ensure a large coverage of the cluster. We add the dierent pointings together and fit elliptical beta-models to the data. We subtract the cluster models from the data and look for residuals, which can be interpreted as substructure. We find several significant structures: the well-known subgroup connected to NGC 4839 in the South-West of the cluster, and another substructure located between NGC 4839 and the centre of the Coma cluster. Constructing a hardness ratio image, which can be used as a temperature map, we see that in front of this new structure the temperature is significantly increased (higher or equal 10 keV). We interpret this temperature enhancement as the result of heating as this structure falls onto the Coma cluster. We furthermore reconfirm the filament-like structure South-East of the cluster centre. This region is significantly cooler than the mean cluster temperature. We estimate the temperature of this structure to be equal or below 1 keV. A possible scenario to explain the observed features is stripping caused by the infall of a small group of galaxies located around the two galaxies NGC 4921 and NGC 4911 into the Coma cluster with a non-zero impact parameter. We also see significant X-ray depressions North and South-East of NGC 4921, which might either be linked to tidal forces due to the merger with the Western structure or connected to an older cluster merger.

XMM-NEWTON DISCOVERY OF AN X-RAY FILAMENT IN COMA

XMM-Newton observations of the outskirts of the Coma cluster of galaxies confirm the existence of warm X-ray gas claimed previously and provide a robust estimate of its temperature (~ 0.2 keV) and oxygen abundance (~ 0.1 solar). Associating this emission with a 20 Mpc infall region in front of Coma, seen in the skewness of its galaxy velocity distribution, yields an estimate of the density of the warm gas of ~ 50 Pcritical . Our measurements of gas mass associated with the warm emission strongly support its nonvirialized nature, suggesting that we are observing the warm-hot intergalactic medium (WHIM). Our measurements provide a direct estimate of the O, Ne and Fe abundance of the WHIM. Differences with the reported Ne/O ratio for some OVI absorbers hints to different origin of the OVI absorbers and the Coma filament.

Wide-field X-ray temperature, pressure, and entropy maps of A754

We present a mosaic of XMM-Newton observations of the nearby major-merger cluster A754 that has either a wider field or better spectral or spatial resolution than previous observations. We construct maps of X-ray surface brightness and temperature integrated along the line of sight. From these two primary maps we derive pseudopressure and pseudoentropy maps. There is structure on a large range of scales in these maps, but the basic pattern is similar to numerical hydrodynamic simulations of cluster mergers. The high surface brightness eastern bar contains gas with the minimum entropy and temperature coupled with the highest iron abundance and density in the cluster. A new feature revealed by these observations is a plumelike structure that appears to emerge from the bar heading northwest. The diffuse radio source also occupies this region, and there is some correspondence between the two. Another new feature is a rim of hot gas to the east, south, and west. We interpret the bar as the core gas from the original main cluster flattened and displaced from the dark matter potential minimum by the merger. The hot rim is the outgoing forward shock from the merger. However, this and previous shocks were weak (M ≤ 2.25), so they are likely only small contributors to the radio-emitting particles. These observations lend support to the merger hypothesis in A754, but some of the parameters of existing models need modification.

The NGC 4839 group falling into the Coma cluster observed by XMM-Newton

We present here the rst analysis of the XMM-Newton EPIC-MOS data of the galaxy group around NGC 4839, which lies at a projected distance to the Coma cluster center of 1.6h 1 50 Mpc. In our analysis, which includes imaging, spectro-imaging and spectroscopy we nd compelling evidence for the sub group being on its rst infall onto the Coma cluster. The complex temperature structure around NGC 4839 is consistent with simulations of galaxies falling into a cluster environment. We see indications of a bow shock and of ram pressure stripping around NGC 4839. Furthermore our data reveal a displacement between NGC 4839 and the center of the hot gas in the group of about 300h 1 50 kpc. With a simple approximation we can explain this displacement by the pressure force originating from the infall, which acts much stronger on the group gas than on the galaxies.

An X-ray temperature map of Abell 754: A major merger

We present the first two-dimensional X-ray temperature map of the rich cluster of galaxies Abell 754. We also present an X-ray surface brightness map with improved spatial resolution and sensitivity compared with previous maps. Both the temperature map and the surface brightness map show that A754 is in the throes of a violent merger; it is probably far from hydrostatic equilibrium.