Jelle S. Kaastra develops the physical interpretation of X-ray radiation spectra from hot astronomical plasma. His review with collaborators explains how ionization equilibrium, spectral lines and continuum radiation constrain the diffuse gas in galaxy clusters and between galaxies.
A spectrum acts as a diagnostic because different ions and transitions respond differently to temperature and other physical conditions. The interpretation requires an atomic model: an observed spectral line is evidence about the emitting plasma, rather than a direct reading of a single universal temperature.[ref]
Thermal radiation processes by Jelle S. Kaastra and others. Abstract: ionization balance, line and continuum processes and practical X-ray diagnostics
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