Emission and Absorption Features of Magnetically Driven Disk Winds in Black Hole X-Ray Binaries

TL;DR

This work addresses how magnetically driven disk winds in BH XRBs shape X-ray spectra, by integrating self-similar MHD wind models with 3D X-ray radiative transfer and photoionization calculations. The authors generate synthetic spectra in the high/soft state, showing that viewing angle governs whether absorption or emission dominates and that scattering can substantially fill in absorption features, reducing their equivalent widths. Key findings include angle-dependent Fe K absorption profiles with distinct line shapes (e.g., Fe26 blue tails) and the emergence of P-Cygni-like features at intermediate angles, all of which depend on wind density and ionization structure. The approach provides a path to constrain wind geometry and dynamics with future high-resolution missions like XRISM Resolve, advancing our understanding of accretion physics in BH XRBs.

Abstract

We investigate accretion disk winds commonly observed in galactic black hole (BH) X-ray binaries (XRB), which manifest as blueshifted absorption features in X-ray spectra. We model these winds as ideal magnetohydrodynamic outflows of hot plasma driven by global magnetic fields threading the accretion disk around the BH. Using Monte Carlo simulations with MONACO, we solve three-dimensional radiative transfer equations to determine the large-scale ionization structure that produces the observed ionic column densities. Focusing on the high/soft state of the BH XRB, where disk emission provides the dominant source of ionizing X-rays, we calculated synthetic spectra showing resonance absorption and scattered emission from ions in various charge states. Our results demonstrate that systems viewed at high polar angles exhibit prominent multi-ion absorption lines with asymmetric profiles, accompanied by P-Cygni-like emission features that partially reproduce the characteristics seen in the observed spectra. This further implies that even a dense disk wind with a high polar angle is unlikely to be saturated due to effective scattering.

Figures (11)

• Figure 1: Three-dimensional rendering of five individual streamlines launched magnetically from different locations in the innermost region where $R_{\mathrm{g}}$ is gravitational radius. On X-Z plane, we show the poloidal distribution of the normalized hydrogen number density (color-coded), $(\log n- \log n_{\mathrm{min}})/(\log n_{\mathrm{max}}- \log n_{\mathrm{min}})$, density contours (dashed) and the poloidal projection of magnetic field lines (thick solid). Y-Z plane shows the distribution of the normalized poloidal wind velocity (color-coded), $(\log vp - \log vp_{\mathrm{min}})/(\log vp_{\mathrm{max}}- \log vp_{\mathrm{min}})$, with arrows.
• Figure 2: (a) Angular density profile $f(\theta)$ (normalized to unity) given by Equation 1 and (b) angular poloidal velocity profile $g_p(\theta)$ (in units of Keplerian velocity at the base of the wind) given by Equation 2. Red dot denotes the position of the Alfven point.
• Figure 3: The shape of the adopted X-ray spectrum of a fiducial high/soft state Shidatsu2019.
• Figure 4: (a) The distribution of the hydrogen number density $\log n_{\mathrm{H}}/\mathrm{cm}^{-3}$. (b) The distribution of the radial velocity $\log v_{\mathrm{out}}/\mathrm{km} \ \mathrm{s}^{-1}$. (c) The distribution of the temperature $\log T/\mathrm{K}$. (d) The distribution of the turbulent velocity $\log v_{\mathrm{turb}}/\mathrm{km} \ \mathrm{s}^{-1}$.
• Figure 5: The distribution of the ionization parameter $\log \xi/\mathrm{erg} \ \mathrm{cm} \ \mathrm{s}^{-1}$.