Thermal radiation processes
  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. 
J.S. Kaastra, F.B.S. Paerels, F. Durret, S. Schindler, P. Richter