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

Efrain Gatuzz, Maxime Parra, Muhammet F. Hasoglu, Thomas W. Gorczyca, Shifra Mandel, Kaya Mori, Kai Matsunaga, Hideki Uchiyama, Masayoshi Nobukawa, Megumi Shidatsu

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.

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

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 J1744294, using high-resolution XRISM Resolve spectra complemented by Chandra HETG data. The analysis employs an updated ISMabs model, incorporating newly computed -matrix photoabsorption cross-sections for Ca ICa 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 - 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.

Paper Structure

This paper contains 15 sections, 4 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Data reduction and continuum modeling
  3. XRISM observation
  4. Chandra observations
  5. Continuum modeling
  6. Spectral fit of the S K-edge
  7. Spectral fit of the Ar K-edge
  8. Spectral fit of the Ca K-edge
  9. Ca K-edge photoabsorption cross-sections
  10. Spectral fit results
  11. Discussion
  12. Sulfur: Comparison with Previous ISM Studies
  13. Argon: Comparison with Previous ISM Studies
  14. Calcium: First Detection of X-ray Absorption in the ISM
  15. Conclusions and Summary

Figures (4)

  • Figure 1: Best-fit results in the S K-edge photoabsorption region for MAXI J1744$-$294. Black points correspond to the observed flux, while the red curve shows the best-fit model. Residuals are displayed in units of $(data-model)/error$. The data has been rebinned for illustrative purposes. The positions of the K$\alpha$ absorption lines are indicated for each ion. The S ii line was shifted by approximately $\Delta E = -8.64$ eV cor25.
  • Figure 2: Best-fit results in the Ar K-edge region for MAXI J1744$-$294. Black points show the observed flux, and the red curve represents the best-fit model. Residuals are displayed in units of $(data-model)/error$, with K$\alpha$ absorption lines indicated for each ion. The data has been rebinned for illustrative purposes.
  • Figure 3: Photoabsorption cross sections included in the model for Ca i (top panel), Ca ii, Ca iii (middle panel), and Ca xviii, Ca xix and Ca xx (bottom panel). For the last ones, our model includes the photoabsorption cross-sections from wit09. For comparison, Ca i-Ca iii photabsorption cross-sections computed by ver95 are also included. The overall profiles are alike, but the K-edge positions vary between the two calculations because this work accounts for orbital-relaxation effects when generating a basis set designed for inner-shell ionization.
  • Figure 4: Best-fit results in the Ca K-edge photoabsorption region for the LMXB MAXI J1744-294. Black points correspond to the observation in flux units, while the black line corresponds to the best-fit model. Residuals are included in units of $(data-model)/error$. The data has been rebinned for illustrative purposes. The positions of the K$\alpha$ absorption lines are indicated for each ion, following the color code used in Figure \ref{['fig_ca_cross']}.