New Benchmark of X-ray Line Emission Models of Fe XVII

TL;DR

This paper evaluates existing Fe XVII X-ray line emission models against Chandra observations, identifies discrepancies due to electron correlation effects, and proposes an improved theoretical approach that enhances agreement with observations and laboratory data.

Contribution

It introduces a new atomic data calculation method incorporating electron correlation effects, improving the accuracy of Fe XVII line emission models.

Findings

Significant discrepancies found between models and observations for Fe XVII line ratios.

Electron correlation effects are crucial for accurate atomic structure calculations.

The new method improves agreement with both astrophysical and laboratory data.

Abstract

We review the accuracy of existing \ion{Fe}{17} X-ray line emission models by comparing them with an extensive analysis of \textit{Chandra} high energy transmission grating (HETG) observations of stellar coronae. We find significant discrepancies between most theoretical predictions and observations for at least some of the intensity ratios involving the six principal Fe XVII lines, 3C (15.01 {\AA}), 3D (15.26 {\AA}), 3E (15.45 {\AA}), 3F (16.78 {\AA}), 3G (17.05 {\AA}), and M2 (17.10 {\AA}). We suggest that the main problem of most previous theoretical studies to their inability to fully include electron correlation effects in the atomic structure calculations, while any deficiencies in the scattering approximation methods are of minor importance, regardless of it being close-coupling (CC) or distorted-wave (DW). An approximate method based on the many-body perturbation theory and DW approximation is proposed to include such correlation effects in the calculation of collisional excitation cross sections. The results are shown to agree with coronal observations and laboratory measurements better than most previous theories. Using the new atomic data, we then investigate the electron density sensitivity of the M2/3G intensity ratio and provide an improved density diagnostic tool for astrophysical observations.