He-like ions as practical astrophysical plasma diagnostics: From stellar coronae to active galactic nuclei

TL;DR

This paper reviews the use of He-like ion line ratios in X-ray spectra as diagnostics for plasma temperature, density, and ionization processes across various astrophysical environments, highlighting recent advances and future prospects.

Contribution

It provides a comprehensive overview of He-like ion diagnostics, extending their application from solar to diverse astrophysical plasmas and discussing upcoming capabilities with next-generation X-ray satellites.

Findings

He-like line ratios determine plasma temperature and density.

Diagnostics have been extended from solar to various astrophysical objects.

Future satellites will resolve higher-Z lines for more detailed plasma analysis.

Abstract

We review X-ray plasma diagnostics based on the line ratios of He-like ions. Triplet/singlet line intensities can be used to determine electronic temperature and density, and were first developed for the study of the solar corona. Since the launches of the X-ray satellites Chandra and XMM-Newton, these diagnostics have been extended and used (from CV to Si XIII) for a wide variety of astrophysical plasmas such as stellar coronae, supernova remnants, solar system objects, active galactic nuclei, and X-ray binaries. Moreover, the intensities of He-like ions can be used to determine the ionization process(es) at work, as well as the distance between the X-ray plasma and the UV emission source for example in hot stars. In the near future thanks to the next generation of X-ray satellites (e.g., Astro-H and IXO), higher-Z He-like lines (e.g., iron) will be resolved, allowing plasmas with higher temperatures and densities to be probed. Moreover, the so-called satellite lines that are formed closed to parent He-like lines, will provide additional valuable diagnostics to determine electronic temperature, ionic fraction, departure from ionization equilibrium and/or from Maxwellian electron distribution.