Published in Space Science Reviews, Volume 134, Issue 1-4, pp. 155-190, 2008.
astro-ph/0801.1011

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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

Table of Contents

• INTRODUCTION
• A SHORT INTRODUCTION TO ATOMIC STRUCTURE
• The Bohr atom
• Level notation in multi-electron systems
• Binding energies
• Abundances
• BASIC PROCESSES
• Excitation processes
• Ionisation processes
• Recombination processes
• Charge transfer processes
• IONISATION BALANCE
• Collisional Ionisation Equilibrium (CIE)
• Non-Equilibrium Ionisation (NEI)
• Photoionisation Equilibrium (PIE)
• EMISSION PROCESSES
• Continuum emission processes
• Line emission processes
• Some important line transitions
• ABSORPTION PROCESSES
• Continuum versus line absorption
• Continuum absorption
• Line absorption
• Some important X-ray absorption lines
• Curve of growth
• Galactic foreground absorption
• GALACTIC FOREGROUND EMISSION
• COOLING FUNCTION
• IONISATION AND EXCITATION BY NON-THERMAL ELECTRONS
• PLASMA MODELLING
• Available codes
• Important parameters
• Multi-Temperature emission and absorption
• REFERENCES