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Fe XVIII-XXIV K beta Inner-shell Absorption Lines in the X-ray Spectra of Neutron Star and Black Hole Binaries with XRISM

Masahiro Tsujimoto, Daiki Miura, Hiroya Yamaguchi, Ehud Behar, Chris Done, Maria Diaz Trigo, Chamani M. Gunasekera, Peter A. M. van Hoof, Stefano Bianchi, Maryam Dehghanian, Gary J. Ferland

TL;DR

The paper tackles the deficit of Fe K beta inner-shell data for mildly ionized Fe in atomic databases, which hinders interpretation of XRISM high-resolution spectra. It performs ab initio atomic-structure calculations with FAC to generate Fe K alpha and K beta data for Fe ions from Li-like to F-like, validates them against ground measurements, and integrates them into the cloudy NLTE radiative transfer framework to synthesize X-ray spectra. The method yields reasonably good agreement with XRISM observations of neutron star and black hole binaries, illustrating the feasibility of ab initio data-driven spectral modeling and the necessity to expand atomic databases. The results provide a path to more accurate plasma diagnostics of accreting systems and outflows and emphasize the impact of improved atomic data on high-resolution X-ray spectroscopy.

Abstract

The advent of the X-ray microcalorimeter spectrometer Resolve onboard the XRISM space telescope opened a new era for high-resolution X-ray spectroscopy of astrophysical plasmas. Many spectral features were newly detected, including the K alpha and K beta inner-shell transition lines of mildly ionized (F- to Li-like) Fe at 6-8 keV in the spectra of X-ray binaries and active galactic nuclei. The widely used atomic databases contain information on the K alpha but not K beta lines of these ions. We conducted the atomic structure calculation using FAC to derive the Fe K alpha and K beta lines and verified the result against ground experiments and other calculations of the Fe K alpha lines. We then implemented the Fe K beta lines in a radiative transfer code (cloudy) and compared the synthesized and observed spectra with XRISM. A reasonably good agreement was obtained between the observation and the ab initio calculations. This exemplifies the need to expand the atomic databases to interpret astrophysical spectra.

Fe XVIII-XXIV K beta Inner-shell Absorption Lines in the X-ray Spectra of Neutron Star and Black Hole Binaries with XRISM

TL;DR

The paper tackles the deficit of Fe K beta inner-shell data for mildly ionized Fe in atomic databases, which hinders interpretation of XRISM high-resolution spectra. It performs ab initio atomic-structure calculations with FAC to generate Fe K alpha and K beta data for Fe ions from Li-like to F-like, validates them against ground measurements, and integrates them into the cloudy NLTE radiative transfer framework to synthesize X-ray spectra. The method yields reasonably good agreement with XRISM observations of neutron star and black hole binaries, illustrating the feasibility of ab initio data-driven spectral modeling and the necessity to expand atomic databases. The results provide a path to more accurate plasma diagnostics of accreting systems and outflows and emphasize the impact of improved atomic data on high-resolution X-ray spectroscopy.

Abstract

The advent of the X-ray microcalorimeter spectrometer Resolve onboard the XRISM space telescope opened a new era for high-resolution X-ray spectroscopy of astrophysical plasmas. Many spectral features were newly detected, including the K alpha and K beta inner-shell transition lines of mildly ionized (F- to Li-like) Fe at 6-8 keV in the spectra of X-ray binaries and active galactic nuclei. The widely used atomic databases contain information on the K alpha but not K beta lines of these ions. We conducted the atomic structure calculation using FAC to derive the Fe K alpha and K beta lines and verified the result against ground experiments and other calculations of the Fe K alpha lines. We then implemented the Fe K beta lines in a radiative transfer code (cloudy) and compared the synthesized and observed spectra with XRISM. A reasonably good agreement was obtained between the observation and the ab initio calculations. This exemplifies the need to expand the atomic databases to interpret astrophysical spectra.
Paper Structure (6 sections, 6 figures)

This paper contains 6 sections, 6 figures.

Table of Contents

  1. Introduction
  2. Atomic Structure Calculation
  3. Calculation
  4. Verification
  5. Radiative Transfer Calculation
  6. Comparison to Observations

Figures (6)

  • Figure 1: XRISM X-ray spectra (continuum-subtracted and normalized) of Cyg X-3 collaboration2024 and Cir X-1tsujimoto2025 for the (a) Fe K$\alpha$ and (b) K$\beta$ features of different charge states.
  • Figure 2: Scatter plot of the line energy ($E$) and the weighted oscillator strengths ($gf$) of the Fe K$\alpha$ and K$\beta$ transitions calculated with FAC. Different symbols are used for the transitions from the ground or a metastable state. Different colors are used for different ionization stages.
  • Figure 3: Scatter plot of the Fe K$\alpha$ line energy of the FAC calculation (this work), Palmeri et al. (2003; P03)palmeri2003a, and chianti version 11.0 delzanna2021 (C11) against a ground experimentrudolph2013. The line labels follow rudolph2013.
  • Figure 4: Scatter plot of relative $E$ and the $A$ value of the Fe K$\alpha$ and K$\beta$ (respectively in smaller and larger symbols) between Palmeri et al. (2003; P03)palmeri2003a and chianti version 11 delzanna2021 (C11) against the FAC calculation (this work).
  • Figure 5: Transmission through the plasma of $n=10^{12.0}$ cm$^{-3}$, $N_{\mathrm{H}}=10^{22.5}$ cm$^{-2}$, and $\xi=10^{2.8}$ erg cm s$^{-1}$ calculated with cloudy by adding the Be- to F-like Fe K$\beta$ transitions (red) compared to the original (black). Line energies of the K$\beta$ transitions are shown by vertical lines separately for those from the GS or a MS with a width proportional to their $\log{(gf)}$ value.
  • ...and 1 more figures