Plasma code for astrophysical charge exchange emission at X-ray wavelengths

TL;DR

This paper introduces a new spectral code for modeling charge exchange X-ray emission, crucial for understanding astrophysical plasma interactions and improving the interpretation of X-ray observations.

Contribution

The authors develop a comprehensive spectral code that approximates atomic cross sections and performs radiative cascade calculations to model charge exchange emission in X-ray wavelengths.

Findings

Successfully reproduces observed spectrum of comet C/2000 WM1 (LINEAR)

Shows charge exchange emission has distinctive spectral features

Enables remote probing of ion properties at plasma interfaces

Abstract

Charge exchange X-ray emission provides unique insights into the interactions between cold and hot astrophysical plasmas. Besides its own profound science, this emission is also technically crucial to all observations in the X-ray band, since charge exchange with the solar wind often contributes a significant foreground component that contaminates the signal of interest. By approximating the cross sections resolved to $n$ and $l$ atomic subshells, and carrying out complete radiative cascade calculation, we create a new spectral code to evaluate the charge exchange emission in the X-ray band. Comparing to collisional thermal emission, charge exchange radiation exhibits enhanced lines from large-$n$ shells to the ground, as well as large forbidden-to-resonance ratios of triplet transitions. Our new model successfully reproduces an observed high-quality spectrum of comet C/2000 WM1 (LINEAR), which emits purely by charge exchange between solar wind ions and cometary neutrals. It demonstrates that a proper charge exchange model will allow us to probe remotely the ion properties, including charge state, dynamics, and composition, at the interface between the cold and hot plasmas.