CONTEXT: Vela X-1, a prototypical high mass X-ray binary (HMXB), hosts a
neutron star (NS) in a close orbit around an early-B supergiant donor star.
Accretion of the donor star's wind onto the NS powers its strong X-ray
luminosity. To understand the physics of HMXBs, detailed knowledge about the
donor star winds is required. AIMS: To gain a realistic picture of the donor
star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model
describing the wind stratification while properly reproducing the observed
donor spectrum. To investigate how X-ray illumination affects the stellar wind,
we calculated additional models for different X-ray luminosity regimes.
METHODS: We use the recently updated version of the PoWR code to consistently
solve the hydrodynamic equation together with the statistical equations and the
radiative transfer. RESULTS: The wind flow in Vela X-1 is driven by ions from
various elements with Fe III and S III leading in the outer wind. The
model-predicted mass-loss rate is in line with earlier empirical studies. The
mass-loss rate is almost unaffected by the presence of the accreting NS in the
wind. The terminal wind velocity is confirmed at v∞≈600 km/s.
On the other hand, the wind velocity in the inner region where the NS is
located is only ≈100 km/s, which is not expected on the basis of a
standard β-velocity law. In models with an enhanced level of X-rays, the
velocity field in the outer wind can be altered. If the X-ray flux is too high,
the acceleration breaks down because the ionization increases. CONCLUSIONS:
Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model
reveals a low wind speed at the NS location, and it provides quantitative
information on wind driving in this important HMXB.Comment: 19 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic