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Session 110 - X-ray, Binaries & Accretion Disks.
Display session, Saturday, January 10
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.
Program listing for Saturday