Probing Accretion Disk Winds of Stratified Nature with Fe XXVI Doublet in Black Hole X-ray Binaries

TL;DR

This work tackles how to diagnose ionized disk winds in BH X-ray binaries by exploiting the Fe XXVI Lyα doublet. It couples a physically motivated stratified, magnetically driven disk wind model with post-process photoionization using XSTAR and radiative-transfer calculations to predict how Fe XXVI doublet profiles depend on wind density, line-of-sight velocity, and turbulent broadening. The key finding is that the doublet morphology can range from broad single-peaked to canonical 1:2 and to nearly 1:1 flux ratios as the local velocity dispersion varies, with XRISM/Resolve simulations showing these features are observationally accessible. This provides a direct diagnostic of wind density and velocity structure in BH XRB winds and offers a pathway to constrain wind physics and launching mechanisms, with potential extensions to AGN winds.

Abstract

Powerful ionized accretion disk winds are often observed during episodic outbursts in Galactic black hole transients. Among those X-ray absorbers, \fexxvi\ doublet structure (Ly$α_1$+Ly$α_2$ with $\sim 20$eV apart) has a unique potential to better probe the underlying physical nature of the wind; i.e. density and kinematics. We demonstrate, based on a physically-motivated magnetic disk wind scenario of a stratified structure in density and velocity, that the doublet line profile can be effectively utilized as a diagnostics to measure wind density and associated velocity dispersion (due to thermal turbulence and/or dynamical shear motion in winds). Our simulated doublet spectra with post-process radiative transfer calculations indicate that the profile can be (1) broad with a single peak for higher velocity dispersion ($\gsim 5,000$ km~s$^{-1}$), (2) a standard shape with 1:2 canonical flux ratio for moderate dispersion ($\sim 1,000-5,000$ km~s$^{-1}$) or (3) double-peaked with its flux ratio approaching 1:1 for lower velocity dispersion ($\lsim 1,000$ km~s$^{-1}$) in optically-thin regime, allowing various line shape. Such a diversity in doublet profile is indeed unambiguously seen in recent observations with XRISM/Resolve at microcalorimeter resolution. We show that some implications inferred from the model will help constrain the local wind physics where \fexxvi\ is predominantly produced in a large-scale, stratified wind.

Figures (4)

• Figure 1: (a) Three-dimensional rendering of three individual streamlines launched magnetically from different locations. On X-Z plane, we show the poloidal distribution of the normalized wind density (color-coded) $\log \tilde{n}(r,\theta)$, density contours (dashed) and the poloidal projection of magnetic field lines (thick solid). Y-Z plane shows the distribution of the normalized poloidal turbulent velocity $\log \tilde{v}_{\rm turb}(r,\theta)$ (color-coded) with its contours (dashed). Toroidal projection of the streamlines is shown on X-Y plane. (b) Intrinsic X-ray ionizing spectrum (relevant to bright soft state of BH XRBs dominated by thermal radiation from the disk) adopted for post-process radiative transfer calculations with xstar in this work ShidatsuDone19Tanimoto25.
• Figure 2: (a) Distribution of Fe xxvi (hydrogen-equivalent) local column $N_H$ per radial bin (dark), cumulative column (blue) and the wind temperature $T$ (red) as a function of ionization parameter $\xi$ (and LoS distance $\log r[\rm cm]$) for $\theta=70^\circ$ (LoS). Vertical dashed line denotes the peak (at $r=r_{\rm peak}$) point where Fe xxvi column is maximum. (b) Calculated doublet structure of Fe xxvi absorption line corresponding to Figure \ref{['fig:f2']}a for various broadening dispersion factor $f_{\rm turb}$. Rest-frame Ly$\alpha_1$ and Ly$\alpha_2$ energies are denoted by vertical lines. We assume $f_{\rm den}=2$ (i.e. $n_o f_{\rm den}=2.7 \times 10^{17}$ cm$^{-3}$), $p=1.2$ and $f_v=0.1$ under optically-thin regime.
• Figure 3: (a) Calculated line ratio (Ly$\alpha_1$/Ly$\alpha_2$) corresponding to Figure \ref{['fig:f2']}b as a function of broadening dispersion $f_{\rm turb}$ with $f_v=0.1$ and $f_{\rm den}=2$ where red line denotes the expected 1:2 ratio under optically-thin regime. Denoted is the corresponding $v_{\rm turb,in}$ value. (b) Simulated 50ks XRISM/ Resolve spectrum with $f_{\rm den}=2$ and $f_v=0.1$ assuming 2-10 keV flux of $10^{-10}$ [cgs] for various dispersion; $f_{\rm turb} = 0.1$ (dark), $1$ (red), $1.3$ (blue) and $4$ (green).
• Figure 4: Similar to Figure \ref{['fig:f2']}b but for various $f_{\rm den}$ ranging from optically-thin regime (e.g. $f_{\rm den} \mathrel{\hbox{$<$}\mkern-14mu \hbox{$\sim$}} 2$ or $N^{\rm peak}_H \mathrel{\hbox{$<$}\mkern-14mu \hbox{$\sim$}} 10^{21}$ cm$^{-2}$ yielding roughly 1:2 ratio) to optically-thick regime (e.g. $f_{\rm den}=10$ or $N^{\rm peak}_H \mathrel{\hbox{$>$}\mkern-14mu \hbox{$\sim$}} 10^{22}$ cm$^{-2}$ leading approximately to 1:1) in the same wind model with $f_v=0.1$ and $f_{\rm turb}=1$. Note that the cumulative column exceeds $\sim 10^{23}$ cm$^{-2}$ for $f_{\rm den} \mathrel{\hbox{$>$}\mkern-14mu \hbox{$\sim$}} 5$.