The stellar wind velocity field of HD 77581

TL;DR

This study uses high-resolution X-ray observations of Vela X-1 to constrain the stellar wind velocity profile of its supergiant companion, revealing faster wind speeds near the surface than previously assumed.

Contribution

It provides the first detailed comparison between hydrodynamic simulations and X-ray observations to determine the wind velocity close to a supergiant star's surface.

Findings

Wind velocity near the surface is twice as large as standard models suggest.

A local velocity gradient of approximately 0.5 is favored.

Hydrodynamic simulations successfully reproduce key observational features.

Abstract

The early acceleration of stellar winds in massive stars is poorly constrained. The scattering of hard X-ray photons emitted by the pulsar in the high-mass X-ray binary Vela X-1 can be used to probe the stellar wind velocity and density profile close to the surface of its supergiant companion HD 77581. We built a high signal-to-noise and high resolution hard X-ray lightcurve of Vela X-1 measured by Swift/BAT over 300 orbital periods of the system and compared it with the predictions of a grid of hydrodynamic simulations. We obtain a very good agreement between observations and simulations for a narrow set of parameters, implying that the wind velocity close to the stellar surface is twice larger than usually assumed with the standard beta law. Locally a velocity gradient of $\beta\sim0.5$ is favoured. Even if still incomplete, hydrodynamic simulations are successfully reproducing several observational properties of Vela X-1.