# The stellar wind velocity field of HD 77581 (Vela X-1 donor)

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 β ∼ 0.5 is favoured. Even if still incomplete, hydrodynamic simulations are successfully reproducing several observational properties of Vela X-1.

Observed hard X-ray
flux (blue points) along the orbit of the system together with simulation results (black line)

The comparison between X-ray data and simulation alows to probe accurately the parameters driving the stellar wind acceleration.

Wind velocity field close to the stellar surface.
The stellar wind velocity increases from the stellar surface (left) to about 1500 km/s at a distance equivalent to one stellar radius (r
ight). The velocity deduced from our observations (black solid line) increases twice faster than usually assumed.