Modeling Magnetic Disk-Wind State Transitions in Black Hole X-ray Binaries

TL;DR

This study models and analyzes the intrinsic state transitions of accretion-disk winds in black hole X-ray binaries using MHD calculations and archival Chandra data, revealing a bi-modal wind density gradient change during state transitions.

Contribution

It introduces a self-consistent MHD-based wind model that explains the bi-modal transition of X-ray winds in black hole binaries, incorporating multi-ion spectra and element abundances.

Findings

Wind density gradient steepens during transition from wind-on to wind-off state.

Ionized wind remains physically present even in wind-off state.

Super-solar abundances of heavier elements are favored.

Abstract

We analyze three prototypical black hole (BH) X-ray binaries (XRBs), \4u1630, \gro1655\ and \h1743, in an effort to systematically understand the intrinsic state transition of the observed accretion-disk winds between \windon\ and \windoff\ states by utilizing state-of-the-art {\it Chandra}/HETGS archival data from multi-epoch observations. We apply our magnetically-driven wind models in the context of magnetohydrodynamic (MHD) calculations to constrain their (1) global density slope ($p$), (2) their density ($n_{17}$) at the foot point of the innermost launching radius and (3) the abundances of heavier elements ($A_{\rm Fe,S,Si}$). Incorporating the MHD winds into {\tt xstar} photoionization calculations in a self-consistent manner, we create a library of synthetic absorption spectra given the observed X-ray continua. Our analysis clearly indicates a characteristic bi-modal transition of multi-ion X-ray winds; i.e. the wind density gradient is found to steepen (from $p \sim 1.2-1.4$ to $\sim 1.4-1.5$) while its density normalization declines as the source transitions from \windon\ to \windoff\ state. The model implies that the ionized wind {\it remains physically present} even in \windoff\ state, despite its absent appearance in the observed spectra. Super-solar abundances for heavier elements are also favored. Our global multi-ion wind models, taking into account soft X-ray ions as well as Fe K absorbers, show that the internal wind condition plays an important role in wind transitions besides photoionization changes. % Simulated {\it XRISM}/Resolve and {\it Athena}/X-IFU spectra are presented to demonstrate a high fidelity of the multi-ion wind model for better understanding of these powerful ionized winds in the coming decades.