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)^β, where β 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 L_X/Ṁ=2.6+/-0.1×10⁴³ ergs s^-1 M^-1_solar yr, where L_X is the X-ray luminosity and Ṁ 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.