The stellar wind velocity field of HD 77581
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The early acceleration of stellar winds in massive stars is poorly constrained. The comparison of X-ray observations of the high-mass X-ray binary Vela X-1 accumulated over 9 years, with hydodynamical simulations allows to constrain the stellar wind velocity field very close to the surface of the super-giant companion. The observed acceletration is more efficient than usually assumed.
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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 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.
Credits: A.Manousakis/R.Walter
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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 (right). The velocity deduced from our observations (black solid line) increases twice faster than usually assumed.
Credits: A.Manousakis/R.Walter
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Observed hard X-ray flux (blue points) along the orbit of the system together with simulation results (black line)
Wind velocity field close to the stellar surface.