Models of X‐Ray Photoionization in LMC X‐4: Slices of a Stellar Wind

Abstract

We show that the orbital variation in the UV P Cygni lines of the X-ray binary LMC X-4 results when X-rays photoionize nearly the entire region outside of the X-ray shadow of the normal star. We fit models to Goddard High Resolution Spectrograph (GHRS) observations of N V and C IV P Cygni line profiles. Analytic methods assuming a spherically symmetric wind show that the wind velocity law is well fitted by v (1 - 1/r)?, where ? is likely ?1.4-1.6 and definitely less than 2.5. Escape probability models can fit the observed P Cygni profiles and provide measurements of the stellar wind parameters. The fits determine LX/ = 2.6 ? 0.1 ? 1043 ergs s-1 M -1? yr, where LX is the X-ray luminosity and is the mass-loss rate of the star. Allowing an inhomogeneous wind improves the fits. IUE spectra show greater P Cygni absorption during the second half of the orbit than during the first. We discuss possible causes of this effect.