The thermal-radiative wind in the neutron star low mass X-ray binary GX 13+1

TL;DR

This study models a thermal-radiative wind in the neutron star binary GX 13+1 using radiation hydrodynamics and Monte Carlo transfer, fitting observed X-ray absorption lines to understand wind structure and turbulence.

Contribution

It presents a comprehensive simulation of the thermal-radiative wind in GX 13+1, constraining turbulence levels and magnetic acceleration contributions based on high-resolution X-ray data.

Findings

The wind is very strong with large ion columns.

Radial turbulence above ~500 km/s is ruled out.

Azimuthal turbulence is limited to ~100 km/s.

Abstract

We fit the observed high ionisation X-ray absorption lines in the neutron star binary GX13+1 with a full simulation of a thermal-radiative wind. This uses a radiation hydrodynamic code coupled to Monte Carlo radiation transfer to compute the observed line profiles from Hydrogen and Helium-like iron and Nickel, including all strong K{\alpha} and K{\beta} transitions. The wind is very strong as this object has a very large disc and is very luminous. The absorption lines from Fe K{\alpha} are strongly saturated as the ion columns are large, so the line equivalent widths (EWs) depend sensitively on the velocity structure. We additionally simulate the lines including isotropic turbulence at the level of the azimuthal and radial velocities. We fit these models to the Fe xxv and xxvi absorption lines seen in the highest resolution Chandra third order HETGS data. These data already rule out the addition of turbulence at the level of the radial velocity of ~500 km/s. The velocity structure predicted by the thermal-radiative wind alone is a fairly good match to the observed profile, with an upper limit to additional turbulence at the level of the azimuthal velocity of ~100 km/s. This gives stringent constraints on any remaining contribution from magnetic acceleration.