Ne X X-ray Emission due to Charge Exchange in M82

TL;DR

This paper investigates Ne X-ray emission in M82, proposing charge exchange as the cause of observed line ratios and constraining ion energies below 500 eV/u through theoretical spectra comparisons.

Contribution

It introduces a detailed analysis of charge exchange processes for Ne X in astrophysical environments, identifying the most reliable computational method for predicting spectra.

Findings

Charge exchange explains the anomalous Ne X line ratios.

Ion kinetic energy in M82 is constrained to <500 eV/u.

The low-energy l-distribution MCLZ method is most reliable for predictions.

Abstract

Recent X-ray observations of star-forming galaxies such as M82 have shown the Ly beta/Ly alpha line ratio of Ne X to be in excess of predictions for thermal electron impact excitation. Here we demonstrate that the observed line ratio may be due to charge exchange and can be used to constrain the ion kinetic energy to be <500 eV/u. This is accomplished by computing spectra and line ratios via a range of theoretical methods and comparing these to experiments with He over astrophysically relevant collision energies. The charge exchange emission spectra calculations were performed for Ne[10+] +H and Ne[10+] +He using widely applied approaches including the atomic orbital close coupling, classical trajectory Monte Carlo, and multichannel Landau- Zener (MCLZ) methods. A comparison of the results from these methods indicates that for the considered energy range and neutrals (H, He) the so-called "low-energy l-distribution" MCLZ method provides the most likely reliable predictions.