XRISM spectroscopy of a crowded Galactic center region -- III. S, Ar and Ca ISM Absorption Features in the spectrum of MAXI J1744-294

TL;DR

This study uses high-resolution X-ray spectra to analyze sulfur, argon, and calcium absorption features in the interstellar medium toward MAXI J1744-294, revealing ionization states, depletion into dust, and consistent hydrogen column densities.

Contribution

First X-ray constraints on calcium absorption in the ISM, employing updated cross-sections and a comprehensive multi-element analysis to probe ISM properties.

Findings

S and Ar are mainly in low-ionization states in the cold and warm ISM.

Calcium is primarily in low-ionization states with evidence of dust depletion.

Hydrogen column densities derived from S, Ar, and Ca are consistent, around 1.1-1.3 x 10^{23} cm^{-2}.

Abstract

We present a comprehensive study of X-ray absorption by sulfur (S), argon (Ar), and calcium (Ca) in the interstellar medium (ISM) along the line of sight to the low-mass X-ray binary MAXI J1744$-$294, using high-resolution XRISM Resolve spectra complemented by Chandra HETG data. The analysis employs an updated ISMabs model, incorporating newly computed $R$-matrix photoabsorption cross-sections for Ca I$-$Ca III, and existing cross-sections for higher ionization states. We find that S and Ar are predominantly in low-ionization states, with S II and Ar II dominating the cold and warm ISM phases, while higher-ionization species are constrained by upper limits. Calcium is primarily detected in low-ionization states, consistent with strong depletion into dust grains, with only marginal contributions from highly ionized ions. Using the measured ionic column densities, we infer hydrogen column densities of $N_{\rm H} \sim 1.1$-$1.3 \times 10^{23}\,\mathrm{cm^{-2}}$ from S and Ar, while the Ca-based value, tracing the neutral ISM, is in agreement with these estimates, highlighting the consistency across different tracers. Our results demonstrate the diagnostic power of combining multiple elements to probe ISM ionization structure, elemental depletion, and dust composition, and provide the first X-ray constraints on calcium absorption in the interstellar medium.