Non-Maxwellian electron distributions in clusters of galaxies

TL;DR

This paper introduces an efficient method to compute X-ray spectra in galaxy clusters considering non-Maxwellian electron distributions, enabling detection of supra-thermal electrons with future high-resolution X-ray instruments.

Contribution

The authors develop a fast, accurate algorithm to model arbitrary electron distributions as Maxwellian sums for X-ray spectral analysis in astrophysical plasmas.

Findings

The method accurately reproduces spectra with non-thermal electrons.

Satellite line intensities are sensitive to supra-thermal electron presence.

Future missions can detect these electrons using high-resolution spectroscopy.

Abstract

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. 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 to calculate the X-ray spectra is needed. We approximate an arbitrary electron distribution by the sum of Maxwellian components. The decomposition is done using either a genetic algorithm or an analytical approximation. The X-ray spectrum is then evaluated using a linear combination of those Maxwellian components. Our method is fast and leads to an accurate evaluation of the spectrum. The use of Maxwellian components allows to use 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 the presence of the 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.