Line Ratios for Solar Wind Charge Exchange with Comets

TL;DR

This paper develops and applies a new set of charge exchange cross sections and line ratios for modeling X-ray emission from comets, improving the accuracy of astrophysical interpretations.

Contribution

It introduces a streamlined computational approach for generating CX cross sections and line ratios, and integrates these into spectral fitting packages for the first time.

Findings

Excellent agreement with XMM-Newton observations of Comet C/2000 WM1.

X-ray intensity mainly from SWCX with hydrogen, with water playing a secondary role.

First implementation of CX cross sections into spectral fitting tools for cometary atmospheres.

Abstract

Charge exchange (CX) has emerged in X-ray emission modeling as a significant process that must be considered in many astrophysical environment--particularly comets. Comets host an interaction between solar wind ions and cometary neutrals to promote solar wind charge exchange (SWCX). X-ray observatories provide astronomers and astrophysicists with data for many X-ray emitting comets that are impossible to accurately model without reliable CX data. Here, we utilize a streamlined set of computer programs that incorporate the multi-channel Landau-Zener theory and a cascade model for X-ray emission to generate cross sections and X-ray line ratios for a variety of bare and non-bare ion single electron capture (SEC) collisions. Namely, we consider collisions between the solar wind constituent bare and H-like ions of C, N, O, Ne, Na, Mg, Al, and Si and the cometary neutrals H2O, CO, CO2, OH, and O. To exemplify the application of this data, we model the X-ray emission of Comet C/2000 WM1 (linear) using the CX package in SPEX and find excellent agreement with observations made with the XMM-Newton RGS detector. Our analyses show that the X-ray intensity is dominated by SWCX with H, while H2O plays a secondary role. This is the first time, to our knowledge, that CX cross sections have been implemented into a X-ray spectral fitting package to determine the H to H2O ratio in cometary atmospheres. The CX data sets are incorporated into the modeling packages SPEX and Kronos.