2015A&A...584A..25M

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 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 higher than usually assumed with the standard beta law. Locally a velocity gradient of β∼0.5 is favoured. Even if still incomplete, hydrodynamic simulations successfully reproduce several observational properties of Vela X-1.