X-RAY ILLUMINATED STELLAR WINDS - IONIZATION EFFECTS IN THE RADIATIVE DRIVING OF STELLAR WINDS IN MASSIVE X-RAY BINARY-SYSTEMS

The X-ray flux in massive X-ray binary systems is produced by the gravitational capture and accretion of material onto a neutron star from the wind of the early-type primary. The X-rays will in turn affect the dynamics of the stellar wind by changing the ionization and temperature structure in the wind and thus altering the radiative force experienced by the wind material. To further our knowledge of massive X-ray binary systems, it is necessary to study how the wind dynamics will be modified when the wind material is illuminated by a strong source of X-rays. In this paper, the effect of X-ray ionization on the radiative force experienced by stellar wind material in a massive X-ray binary system is calculated. The radiative line force from the radiation field of the primary can be parameterized in terms of the Castor, Abbott, and Klein force multiplier M(t). Results are presented for the variation of the line force multiplier M(t) with the ionization parameter ξ (a measure of the degree of photoionization). These show that the line force decreases sharply with increasing X-ray ionization, though in a very nonlinear way. Some of the dynamical consequences of these results are briefly discussed, finding that the wind velocity can be severely decreased in the supersonic portion of the flow by X-ray ionization.