Silicon, sulfur and iron in the interstellar medium: a high-resolution X-ray spectral study of GX 340+0

TL;DR

This study uses high-resolution X-ray spectroscopy to analyze the composition and physical state of dust and gas in the interstellar medium along the line of sight to GX 340+0, revealing detailed dust mineralogy and elemental associations.

Contribution

It provides a detailed characterization of dust and gas in dense ISM regions using simultaneous fitting of multiple absorption edges, improving understanding of dust composition and elemental distribution.

Findings

Amorphous olivine dominates dust composition (~65%)

Approximately 74% of Fe is in silicates, 8% in sulfides, 18% in metallic iron

Detected S II absorption indicating a sulfur dust fraction of ~35%

Abstract

High-resolution X-ray spectroscopy provides a powerful probe of the interstellar medium (ISM), giving direct access to the composition and physical state of dust grains and atomic species in dense environments. We present a study of the gas and dust along the line of sight to the bright low-mass X-ray binary GX 340+0, which samples higher-density gases in the ISM. Using deep Chandra/HETG spectra, we characterize X-ray absorption fine structure from dust, gas absorption lines, and the optical depths of the Si, S, and Fe K-edges. By fitting these three edges simultaneously, we reduce degeneracies in the dust composition and find that amorphous olivine dominates the fractional contribution among the dust columns ($\sim$65%), followed by metallic iron ($\sim$19%), iron sulfides (pyrrhotite and troilite; $\sim$10%), and fayalite ($\sim$5%), with the remaining species contributing only a few percent in total. From the inferred stoichiometry, we estimate that $\sim$74% of Fe is associated with silicates, $\sim$8% with sulfides, and $\sim$18% with metallic iron, suggesting that Fe is predominantly incorporated in iron rich silicate grains along this sightline. We detect S ii absorption and infer a sulfur dust fraction of $\sim$35%. We also detect absorption structure near the Ca and Ar K edges, highlighting the need for improved atomic photoabsorption data. The Chandra/HETG spectral resolution remains essential to disentangle dust and gas contributions at the Si and S K edges, providing a benchmark for dust characterization in high-density regions in the ISM.