Non-Maxwellian electron distributions in clusters of galaxies

Context. Thermal X-ray spectra of clusters of galaxies and other sources are commonly calculated assuming Maxwellian electron distributions. There are situations where this approximation is not valid, for instance near interfaces of hot and cold gas and near shocks. Aims. The presence of non-thermal electrons affects the X-ray spectrum. To study the role of these electrons in clusters and other environments, an efficient algorithm is needed to calculate the X-ray spectra. Methods. We approximate an arbitrary electron distribution by the sum of Maxwellian components. The decomposition uses either a genetic algorithm or an analytical approximation. The X-ray spectrum is then evaluated with a linear combination of those Maxwellian components. Results. Our method is fast and leads to an accurate evaluation of the spectrum. The Maxwellian components allow the use of the standard collisional rates that are available in plasma codes such as SPEX. We give an example of a spectrum for the supra-thermal electron distribution behind a shock in a cluster of galaxies. The relative intensities of the satellite lines in such a spectrum are sensitive to supra-thermal electrons. These lines can only be investigated with high spectral resolution. We show that the instruments on future missions like Astro-H and IXO will be able to demonstrate the presence or absence of these supra-thermal electrons.