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We discuss the different physical processes that are important to understand the thermal X-ray emission and absorption spectra of the diffuse gas in clusters of galaxies and the warm-hot intergalactic medium. The ionisation balance, line and continuum emission and absorption properties are reviewed and several practical examples are given that illustrate the most important diagnostic features in the X-ray spectra. [ABSTRACT FROM AUTHOR]

The Warm-Hot Intergalactic Medium (WHIM) arises from shock-heated gas collapsing in large-scale filaments and probably harbours a substantial fraction of the baryons in the local Universe. Absorption-line measurements in the ultraviolet (UV) and in the X-ray band currently represent the best method to study the WHIM at low redshifts. We here describe the physical properties of the WHIM and the concepts behind WHIM absorption line measurements of H i and high ions such as O vi, O vii, and O viii in the far-ultraviolet and X-ray band. We review results of recent WHIM absorption line studies carried out with UV and X-ray satellites such as FUSE, HST, Chandra, and XMM-Newton and discuss their implications for our knowledge of the WHIM. [ABSTRACT FROM AUTHOR]

The Warm-Hot Intergalactic Medium (WHIM) is thought to contribute about 40–50% to the baryonic budget at the present evolution stage of the universe. The observed large scale structure is likely to be due to gravitational growth of density fluctuations in the post-inflation era. The evolving cosmic web is governed by non-linear gravitational growth of the initially weak density fluctuations in the dark energy dominated cosmology. Non-linear structure formation, accretion and merging processes, star forming and AGN activity produce gas shocks in the WHIM. Shock waves are converting a fraction of the gravitation power to thermal and non-thermal emission of baryonic/leptonic matter. They provide the most likely way to power the luminous matter in the WHIM. The plasma shocks in the WHIM are expected to be collisionless. Collisionless shocks produce a highly non-equilibrium state with anisotropic temperatures and a large differences in ion and electron temperatures. We discuss the ion and electron heating by the collisionless shocks and then review the plasma processes responsible for the Coulomb equilibration and collisional ionisation equilibrium of oxygen ions in the WHIM. MHD-turbulence produced by the strong collisionless shocks could provide a sizeable non-thermal contribution to the observed Doppler parameter of the UV line spectra of the WHIM. [ABSTRACT FROM AUTHOR]

We present the results of a long (∼93 ks)XMM–Newtonobservation of the bright BL-Lac object. OurReflection Grating Spectrometer(RGS) spectrum shows a single absorption feature at an observed wavelength, which we interpret as O viKα absorption at, i.e.∼3000 km s−1 from the background object. The observed equivalent width of the absorption line,∼30 mÅ, coupled with the lack of the corresponding absorption edge in the EPIC pn data, implies a column density ofand turbulence with a Doppler velocity parameter. Within the limitations of our RGS spectrum, no O viior O vKα absorption are detected. Under the assumption of ionization equilibrium by both collisions and the extragalactic background, this is only marginally consistent if the gas temperature is, with significantly lower or higher values being excluded by our limits on O vor O vii. If confirmed, this would be the first X-ray detection of a large amount of intervening warm absorbing gas through O viabsorption. The existence of such a high column density absorber, much stronger than any previously detected one in O vi, would place stringent constraints on the large-scale distribution of baryonic gas in the Universe. [ABSTRACT FROM AUTHOR]

Context. XMM-Newton was launched on 10 December 1999 and has been operational since early 2000. One of the instruments onboard XMM-Newton is the reflection grating spectrometer (RGS). Two identical RGS instruments are available, with each RGS combining a reflection grating assembly and a camera with charge-coupled devices to record the spectra. Aims. We describe the calibration and in-orbit performance of the RGS instrument. By combining the preflight calibration with appropriate inflight calibration data including the changes in detector performance over time, we aim at profound knowledge about the accuracy in the calibration. This will be crucial for any correct scientific interpretation of spectral features for a wide variety of objects. Methods. Ground calibrations alone are not able to fully characterize the instrument. Dedicated inflight measurements and constant monitoring are essential for a full understanding of the instrument and the variations of the instrument response over time. Physical models of the instrument are tuned to agree with calibration measurements and are the basis from which the actual instrument response can be interpolated over the full parameter space. Results. Uncertainties in the instrument response have been reduced to <10% for the effective area and <6 mÅ for the wavelength scale (in the range from 8 Å to 34 Å). The remaining systematic uncertainty in the detection of weak absorption features has been estimated to be 1.5%. Conclusions. Based on a large set of inflight calibration data and comparison with other instruments onboard XMM-Newton, the calibration accuracy of the RGS instrument has been improved considerably over the preflight calibrations. [ABSTRACT FROM AUTHOR]

We present the work of an international team at the International Space Science Institute (ISSI) in Bern that worked together to review the current observational and theoretical status of the non-virialised X-ray emission components in clusters of galaxies. The subject is important for the study of large-scale hierarchical structure formation and to shed light on the “missing baryon” problem. The topics of the team work include thermal emission and absorption from the warm-hot intergalactic medium, non-thermal X-ray emission in clusters of galaxies, physical processes and chemical enrichment of this medium and clusters of galaxies, and the relationship between all these processes. One of the main goals of the team is to write and discuss a series of review papers on this subject. These reviews are intended as introductory text and reference for scientists wishing to work actively in this field. The team consists of sixteen experts in observations, theory and numerical simulations. [ABSTRACT FROM AUTHOR]