Published in Space Science Reviews, Volume 134, Issue 1-4, pp. 155-190, 2008.

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J.S. Kaastra1, F.B.S. Paerels2, F. Durret3, S. Schindler4 and P. Richter5

1 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, Netherlands
Astronomical Institute, Utrecht University P.O. Box 80000, 3508 TA Utrecht, The Netherlands
2 Department of Astronomy, Columbia University, New York, NY, USA
3 Institut d'Astrophysique de Paris, CNRS, UMR 7095, Université Pierre et Marie Curie, 98bis Bd Arago, F-75014 Paris, France
4 Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
5 Physics Institute, Potsdam University, Am Neuen Palais 10, D-14469 Potsdam, Germany

Abstract. 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.

Key words: atomic processes - radiation mechanisms: thermal - intergalactic medium - X-rays:general

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