Compton Scattering of Thermal Disk Radiation with Black Hole Disk Winds

TL;DR

This study investigates how powerful disk winds in black hole X-ray binaries can significantly scatter and alter the thermal disk radiation spectra, impacting spectral analysis and black hole spin measurements.

Contribution

It introduces multi-dimensional Monte Carlo simulations to model Compton scattering of disk radiation by winds, revealing substantial spectral deviations from traditional models.

Findings

Disk winds can down-scatter thermal spectra by up to 70%.

Conventional spectral hardening models may be insufficient in wind-affected systems.

Black hole spin measurements could be significantly affected by wind scattering.

Abstract

Galactic black hole (BH) X-ray binaries are known to exhibit episodic outbursts, during which accretion and spectral mode distinctively transition between low/hard and high/soft state. X-ray observations during high/soft state occasionally reveal a pronounced presence of a powerful disk wind in these systems. However, it is unexplored to date how such winds may influence disk emission in that regime. In this work, we consider an observational implication by Compton scattering of thermal disk radiation due to accretion disk winds by performing multi-dimensional Monte Carlo simulations in the context of a stratified wind of large solid angle launched over a large radial extent of the disk. Compton-scattered thermal disk spectrum is computed for a different wind property; i.e. wind density and its radial gradient. We find that the intrinsic disk radiation can be significantly down-scattered by winds of moderate-to-high density to the extent that the transmitted spectrum can substantially deviate from the conventional multi-color-disk emission in a tangible way. % by factor of up to 70\%. We thus claim that the conventional treatment of spectral hardening in the disk atmosphere may be insufficient to fully account for the observed disk continuum in the presence of strong wind scattering. It is suggested that the effect of scattering process (by $f_w$) should be incorporated to accurately evaluate an intrinsic disk spectrum besides the conventional hardening (color correction) factor (by $f_c$). We argue that BH spin measurements using thermal continuum-fitting in transient XRBs may well be mildly (if not significantly) altered by such spectral "contamination".