1996A&A...311..793F

Light curves of the X-ray pulsar Vela X-1 obtained with the ASCA satellite show a strong asymmetry in the hard energy band during the eclipse of the X-ray source: a steep drop at ingress is followed by a gradual decline, whereas only a steep increase is observed at egress. X-ray scattering off the dense accretion wake trailing the neutron star cannot explain the gradual decline because of the long persistence of the latter ({DELTA}φ=0.11). Instead we propose that scattering in an extended photoionization wake is responsible. This wake is caused by the switch-off of the radiative force that drives the B supergiant wind during the passage through the highly ionized Stroemgren region surrounding the X-ray source. The stalled gas then trails the neutron star, which moves relative to the B star surface (no corotation). A model for the Vela X-1 system which assumes that the B star does not rotate gives too large a phase extent for the dense wake and can be ruled out. Including the B star rotation in an approximate way, the relative azimuthal motion of the neutron star is slower and the wake covers a smaller phase interval. Finally, we assume that the Stroemgren sphere does not reach too deep into the wind accelerating region. The wind can then reach a certain fraction of the terminal velocity before the radiative force is switched off. This elongates the photoionization wake further and both its phase extent and its scattering efficiency match the observations.