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Session 110 - X-ray, Binaries & Accretion Disks.
Display session, Saturday, January 10
Exhibit Hall,
As an X-ray source orbits an OB star with a strong stellar wind, the X-rays will photoionize a region of the wind that appears at varying projected velocities. The resulting change in the wind-formed P Cygni line profiles can be used as a probe of the stellar wind and of the X-ray emission. In the X-ray binary LMC X-4, which consists of an O7-9 III-V star and a neutron star in a 1.4 day orbit, we show that the X-rays ionize nearly the entire region outside of the X-ray shadow of the normal star. As the neutron star orbits the O star, most of the P Cygni line variation results from the moving X-ray shadow of the normal star. Thus the line change between closely spaced orbital phases results from ``slices'' of the stellar wind. We fit theoretical models of X-ray photoionization of the wind to observations of N V and C IV P Cygni line variations taken with the Goddard High Resolution Spectrograph (GHRS) aboard HST (Vrtilek et al. 1997). Time-resolved observations show a rapid change in the P Cygni absorption as the X-ray shadow of the normal star advances across the line of sight. Simple analytical models of this effect allow us to infer how the wind expansion velocity depends on radius.
