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

TL;DR

This paper demonstrates how the Fe XXVI doublet line profile can be used as a diagnostic tool to probe the density and velocity structure of stratified accretion disk winds in black hole X-ray binaries, with implications for understanding wind physics.

Contribution

It introduces a physically-motivated model linking Fe XXVI doublet profiles to wind density and velocity dispersion, validated by simulated spectra and recent observations.

Findings

Doublet profile shapes vary with velocity dispersion, from broad single peaks to double peaks.

The flux ratio of the doublet lines changes with wind conditions, indicating different optical depths.

Recent XRISM observations show diverse doublet profiles consistent with the model.

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$\alpha_1$+Ly$\alpha_2$ 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.