THE X-RAY ABSORPTION-SPECTRUM OF VELA X-1

We present a spectral analysis of all the EXOSAT observations of the massive X-ray binary Vela X-1 (4U 0900-40/HD 77581). The absorbing column density shows erratic factor of 10 increases up to 5 × 1023 H cm-2 that can occur at any orbital phase and last from minutes to many hours. These variations occur on top of a base level of absorption that increases from 3 × 1022 H cm-2 to 15 × 1022 H cm-2 from orbital phase, φ, ∼0.2 to 0.85. The increase in basal absorption after superior conjunction of the X-ray source probably results from the presence of a dense gas stream trailing the neutron star. We suggest the erratic changes in column density are caused by the intermittent formation of a tenuous accretion disk around the neutron star. The X-ray spectra show a low-energy excess above that expected from cold photoelectric absorption. The character of the low-energy excess depends on orbital phase and two different modifications to the simple absorption model are required. Before φ ∼ 0.5, when the overall absorption is a minimum, the effects of X-ray ionization of the wind can model the excess. When the column density is higher, both after φ ∼ 0.5 and during the erratic absorption events, the low-energy excess is too strong to be modeled as ionization of the wind. It can be well modeled as X-rays scattered by the ambient wind around an absorbing medium that does not completely surround the X-ray source. The X-ray scattering model is confirmed by the fact that during intervals of high absorption the low-energy excess is not pulsed.