Satellite lines from autoionizing states of Fe XVI and the problems with the X-ray Fe XVII lines

TL;DR

This paper presents new atomic data calculations for Fe XVI autoionizing states, compares them with previous results, and improves spectral line identifications, resolving discrepancies in solar X-ray observations of Fe lines.

Contribution

It provides updated atomic data and line identifications for Fe XVI and Fe XVII, improving spectral modeling and resolving previous inconsistencies.

Findings

Autoionization rates show high sensitivity to configuration interaction.

New model agrees well with high-resolution solar spectra.

Enhanced flux in 15-15.7 Å range resolves previous observational discrepancies.

Abstract

We present new calculations of atomic data needed to model autoionizing states of Fe XVI. We compare the state energies, radiative and excitation data with a sample of results from previous literature. We find a large scatter of results, the most significant ones in the autoionization rates, which are very sensitive to the configuration interaction and state mixing. We find relatively good agreement between the autoionization rates and the collisional excitation rates calculated with the R-matrix suite of programs and autostructure. The largest model, which includes J-resolved states up to n=10, produces ab-initio wavelengths and intensities of the satellite lines which agree well with solar high-resolution spectra of active regions, with few minor wavelength adjustements. We review previous literature, finding many incorrect identifications, most notably those in the NIST database. We provide several new tentative identifications in the 15-15.7 A range, and several new ones at shorter wavelengths, where previous lines were unidentified. Compared to the previous CHIANTI model, the present one has an increased flux in the 15--15.7 A range at 2 MK of a factor of 1.9, resolving the discrepancies found in the analysis of the Marshall Grazing Incidence X-Ray Spectrometer (MaGIXS) observation. It appears that the satellite lines also resolve the long-standing discrepancy in the intensity of the important Fe XVII 3D line at 15.26 A.