Probing Accretion Disk Winds of Stratified Nature with Fe <scp>xxvi</scp> Doublet in Black Hole X-Ray Binaries

Abstract Powerful ionized accretion disk winds are often observed during episodic outbursts in Galactic black hole transients. Among those X-ray absorbers, Fe xxvi doublet structure (Ly α 1 +Ly α 2 with ∼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 diagnostic to measure wind density and associated velocity dispersion (due to thermal turbulence and/or dynamical shear motion in winds). Our simulated doublet spectra with postprocess radiative transfer calculations indicate that the profile can be (1) broad with a single peak for higher-velocity dispersion (≳5000 km s −1 ), (2) a standard shape with 1:2 canonical flux ratio for moderate dispersion (∼1000–5000 km s −1 ), or (3) double-peaked with its flux ratio approaching 1:1 for lower-velocity dispersion (≲1000 km s −1 ) in an optically thin regime, allowing various line shapes. Such a diversity in doublet profiles 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 Fe xxvi is predominantly produced in a large-scale, stratified wind.