VELA X-1 - HOW TO PRODUCE ASYMMETRIC ECLIPSES

Light curves of the X-ray pulsar Vela X-l obtained with the ASCA satel lite show a strong asymmetry in the hard energy band during the eclips e of the X-ray source: a steep drop at ingress is followed by a gradua l decline, whereas only a steep increase is observed at egress. X-ray scattering off the dense accretion wake trailing the neutron star cann ot explain the gradual decline because of the long persistence of the latter (Delta phi = 0.11). Instead we propose that scattering in an ex tended photoionization wake is responsible. This wake is caused by the switch-off of the radiative force that drives the B supergiant wind d uring the passage through the highly ionized Stromgren region surround ing the X-ray source. The stalled gas then trails the neutron star, wh ich moves relative to the B star surface (no corotation). A model for the Vela X-l system which assumes that the B star does not rotate give s too large a phase extent for the dense wake and can be ruled out. In cluding the B star rotation in an approximate way, the relative azimut hal motion of the neutron star is slower and the wake covers a smaller phase interval. Finally, we assume that the Stromgren sphere does not reach too deep into the wind accelerating region. The wind can then r each 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 obs ervations.