XRISM Insights For Interstellar Sulfur
The X-ray Imaging Spectroscopy Mission (XRISM) provides the best spectral resolution with which to study Sulfur (S) K-shell photoabsorption features from the interstellar medium (ISM).
For the first time, we demonstrate the high-signal detection of interstellar atomic SII K-beta absorption in the spectrum of X-ray binaries (XRBs) 4U 1630-472 and GX 340+0. The persistence of this feature across multiple instruments, targets, and flux states implies that it is interstellar in nature. We measure the SII Kbeta line centroid at 2470.8 +/- 1.1 eV after including systematic uncertainties.
We also find that the most recently published high resolution SII absorption template requires a systematic energy scale shift of +7-8 eV, which is comparable to the level of disagreement among various atomic modeling procedures. The XRISM 300 ks observation of GX 340+0 provides unprecedented signal-to-noise in the S K region, and we find evidence of residual absorption from solid S in the spectra of GX 340+0.
Absorption templates from three Fe-S compounds, troilite (FeS), pyrrhotite (Fe_7S_8) and pyrite (FeS_2), provide equally good fits to the residuals. Even though we are not able to distinguish among these three compounds, they provide equal estimates for the abundance of S locked in dust grains. Having accounted for both the gaseous and solid S in the GX 340+0 sightline provides us with a direct measurement of S depletion, which is 40% +/- 15%.
Our depletion measurement provides an upper limit to the fraction of interstellar Fe bound in Fe-S compounds of < 25%, which is consistent with prior studies of Fe-S compounds via Fe L-shell absorption. Both XRBs in this study are at a distance of approximately 11 kpc and on the opposite side of the Galactic disk, suggesting that this value could represent the average S depletion of the Milky Way when integrated across all phases of the ISM.
Three simultaneous fits to the two XRISM observations for S K-shell photoelectric absorption in the sight line of GX 340+0 with increasing model complexity. The spectra are evaluated separately but plotted as a combined spectrum for visual clarity. (Top:) A parametric description of SII absorption using a Voigt profile for the Kβ absorption line and the XSPEC edge model for the photoelectric absorption continuum. (Middle:) A fit with the SII absorption cross-section presented in Gatuzz et al. (2024), when allowing for a shift in the energy scale. The proposed blueshift of 7.9 eV comes from this dataset. (Bottom:) A fit with the SII template (green solid curve) and absorption from troilite (FeS) dust (blue dashed curve). The change in Bayes Information Criterion relative to the parametric fit (top) is given in each panel, where a positive ∆BIC indicates a more preferred model. — astro-ph.GA
Lia Corrales, Elisa Costantini, Sascha Zeeger, Liyi Gu, Hiromitsu Takahashi, David Moutard, Megumi Shidatsu, Jon M. Miller, Misaki Mizumoto, Randall K. Smith, Ralf Ballhausen, Priyanka Chakraborty, Marua Diaz Trigo, Renee Ludlam, Takao Nakagawa, Ioanna Psaradaki, Shinya Yamada, Caroline A. Kilbourne
Comments: accepted for publication in PASJ
Subjects: Astrophysics of Galaxies (astro-ph.GA); High Energy Astrophysical Phenomena (astro-ph.HE)
Cite as: arXiv:2506.08751 [astro-ph.GA] (or arXiv:2506.08751v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2506.08751
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Submission history
From: Lia Corrales
[v1] Tue, 10 Jun 2025 12:45:43 UTC (122 KB)
https://arxiv.org/abs/2506.08751
Astrobiology, Astrochemistry,
