# The luminosity dependence of thermally-driven disc winds in low-mass X-ray binaries

## TL;DR

This study uses radiation-hydrodynamic simulations to explore how thermally-driven disc winds in low-mass X-ray binaries depend on luminosity, confirming key observational features and providing synthetic absorption line profiles.

## Contribution

It presents detailed simulations covering a wide luminosity range, incorporating radiative transfer, and confirms previous findings while offering new synthetic spectral predictions.

## Key findings

- Wind velocity increases with luminosity.
- Observable absorption features are seen along high-column equatorial sightlines.
- Wind efficiency remains roughly constant at about twice the accretion rate.

## Abstract

We have carried out radiation-hydrodynamic simulations of thermally-driven accretion disc winds in low-mass X-ray binaries. Our main goal is to study the luminosity dependence of these outflows and compare with observations. The simulations span the range $\rm{0.04 \leq L_{acc}/L_{Edd} \leq 1.0}$ and therefore cover most of the parameter space in which disc winds have been observed. Using a detailed Monte Carlo treatment of ionization and radiative transfer, we confirm two key results found in earlier simulations that were carried out in the optically thin limit: (i) the wind velocity -- and hence the maximum blueshift seen in wind-formed absorption lines -- increases with luminosity; (ii) the large-scale wind geometry is quasi-spherical, but observable absorption features are preferentially produced along high-column equatorial sightlines. In addition, we find that (iii) the wind efficiency always remains approximately constant at $\rm{\dot{M}_{wind}/\dot{M}_{acc} \simeq 2}$, a behaviour that is consistent with observations. We also present synthetic Fe XXV and Fe XXVI absorption line profiles for our simulated disc winds in order to illustrate the observational implications of our results.