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The Feasibility of Using Fe XXIII Metastable Transitions as a Density Diagnostic for LMXB Disk Winds

D. L. Moutard, L. R. Corrales, R. Tomaru, C. Done, J. Neilsen, E. Behar, E. Costantini, M. Díaz-Trigo, S. Yamada

TL;DR

This work tackles the challenge of measuring the density of disk winds in low-mass X-ray binaries (LMXBs) to constrain wind location and energetics. It tests a density diagnostic based on metastable transitions of Fe XXIII near 6.61–6.64 keV by applying the photoionization code PION to three representative SEDs (GX 13$+$1, 4U 1735$-$44, and MAXI J1820$+$070) across a grid of ionization parameters and densities. The key finding is that this diagnostic is feasible in the regime $\log{(\xi/\mathrm{erg\,cm\,s^{-1}})} \sim 2-3$ with $\log{(n_H/\mathrm{cm^{-3}})} \gtrsim 14$, with black-hole LMXBs offering the most favorable conditions due to typical ionization states; contamination from Fe XXIV remains a challenge near similar ionization. The authors argue that high-resolution, high signal-to-noise observations with XRISM Resolve could exploit these metastable features to constrain wind densities, providing a valuable tool for mapping the geometry and energetics of LMXB winds.

Abstract

Low mass X-ray binaries (LMXBs) occasionally show signs of outflowing material from the accretion disk. Studying these outflows can inform the understanding of the geometry of the systems, as well as the dynamics and energetics of accretion. One key variable for determining the location of these disk winds is the density of the outflowing material. In this paper we explore a density diagnostic based upon the absorption of ionizing photons by density-sensitive metastable states of Fe XXIII. This can yield a blue shifted complex of absorption features in the region of $6.61-6.64$ keV. We use the photoionization code {\sc pion} to test how varying the ionizing spectrum affects the detectability and interpretation of these features. We base these ionizing spectral energy distributions on GX~13$+$1 to represent a bright thermally dominated spectrum; 4U 1735$-$44 representing a harder, fainter LMXB spectrum; and MAXI J1820$+$070 representing a black hole LMXB spectrum completely dominated by Comptonized emission. For each of these, we find that the regime where Fe XXIII can be used as a density diagnostic is with an ionization parameter $\log{(ξ/{\rm erg~cm~s^{-1}})}\sim2-3$ and an outflow density $\log{(n_H/{\rm cm^{-3})}}\gtrsim14$. The typical range of ionization parameters for LMXBs indicates that this technique is more feasibly achieved with BH LMXBs than their NS counterparts.

The Feasibility of Using Fe XXIII Metastable Transitions as a Density Diagnostic for LMXB Disk Winds

TL;DR

This work tackles the challenge of measuring the density of disk winds in low-mass X-ray binaries (LMXBs) to constrain wind location and energetics. It tests a density diagnostic based on metastable transitions of Fe XXIII near 6.61–6.64 keV by applying the photoionization code PION to three representative SEDs (GX 131, 4U 173544, and MAXI J1820070) across a grid of ionization parameters and densities. The key finding is that this diagnostic is feasible in the regime with , with black-hole LMXBs offering the most favorable conditions due to typical ionization states; contamination from Fe XXIV remains a challenge near similar ionization. The authors argue that high-resolution, high signal-to-noise observations with XRISM Resolve could exploit these metastable features to constrain wind densities, providing a valuable tool for mapping the geometry and energetics of LMXB winds.

Abstract

Low mass X-ray binaries (LMXBs) occasionally show signs of outflowing material from the accretion disk. Studying these outflows can inform the understanding of the geometry of the systems, as well as the dynamics and energetics of accretion. One key variable for determining the location of these disk winds is the density of the outflowing material. In this paper we explore a density diagnostic based upon the absorption of ionizing photons by density-sensitive metastable states of Fe XXIII. This can yield a blue shifted complex of absorption features in the region of keV. We use the photoionization code {\sc pion} to test how varying the ionizing spectrum affects the detectability and interpretation of these features. We base these ionizing spectral energy distributions on GX~131 to represent a bright thermally dominated spectrum; 4U 173544 representing a harder, fainter LMXB spectrum; and MAXI J1820070 representing a black hole LMXB spectrum completely dominated by Comptonized emission. For each of these, we find that the regime where Fe XXIII can be used as a density diagnostic is with an ionization parameter and an outflow density . The typical range of ionization parameters for LMXBs indicates that this technique is more feasibly achieved with BH LMXBs than their NS counterparts.

Paper Structure

This paper contains 12 sections, 2 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Modeling and Methods
  3. Sources used for SED generation
  4. LMXB1: GX 13$+$1
  5. LMXB2: 4U 1735$-$44
  6. LMXB3: MAXI J1820$+$070
  7. PION Model Grid
  8. Discussion
  9. Fe K$\alpha$ Reflection
  10. Feature Depth and Observability
  11. Contamination from Fe XXIV
  12. Summary and Conclusions

Figures (4)

  • Figure 1: The three representative LMXB SEDs defined in the range from 0.01 to 30 keV. The SEDs with a power law component (LMXB2 and LMXB3) show more flux in the softest bands, but the LMXB1 SED has a higher overall luminosity. The flux excess in bands below $\sim$0.1 keV has little effect on the population of ions producing absorption features above 6.5 keV.
  • Figure 2: The relative ion fraction of Fe XXIII and XXIV for various ionizing LMXB SEDs. From this plot, it is clear that the nature of the SED has a strong effect on the concentrations of relevant ions.
  • Figure 3: (a) The solid lines represent relative occupation ratios of the density sensitive metastable states of Fe XXIII for an optically thick ($\log {(N_{\rm H}/{\rm cm}^2)} = 24.11$) wind using the LMXB1 SED. Dashed lines indicate the occupation with significantly lower column density ($\log{(N_{\rm H}/{\rm cm}^2)}=22$ cm$^{-2}$). We see that even with a severe reduction in $N_{\rm H}$, the overall shape remains, with differences apparent at densities $\log n_H \gtrsim 14$. This indicates that, while the column density can affect the population of metastable states, the primary driver is the wind density. (b) and (c) are the same, but for LMXB2 and LMXB3 respectively. We cut the plots at a density of $\log (n_H/\mathrm{cm}^{-3}) \sim 17$, due to sudden breaks in the calculated occupations at that density. These are an indication that spex has found a new solution for the occupations above those densities, and we do not consider these solutions to be physical. As mentioned in Table \ref{['tab:lines']} and Section \ref{['sec:intro']}
  • Figure 4: (a) For LMXB1, we display the optical depth at line center $\tau_0$ as calculated by Equation \ref{['eq:depth']} for the combined $^3P_1$ and $^3P_2$ feature of Fe XXIII. The contours represent the ratio of $\tau_0$ for the features found at $\sim6.617$ keV for Fe XXIII and XXIV respectively (i.e., a value $> 1$ indicates the Fe XXIII feature is stronger than the contaminating Fe XXIV). We use black, grey, and white contours to represent a ratio of 10, 1.0, and 0.1, respectively. (b) and (c) are the same, but for LMXB2 and LMXB3 respectively.