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

We show that the orbital variation in the UV P Cygni lines of the X-raybinary LMC X-4 results when X-rays photoionize nearly the entire region outsideof the X-ray shadow of the normal star. We fit models to HST GHRS observationsof N V and C IV P Cygni line profiles. Analytic methods assuming a sphericallysymmetric wind show that the wind velocity law is well-fit by v~(1-1/r)^beta,where beta is likely 1.4-1.6 and definitely <2.5. Escape probability models canfit the observed P Cygni profiles, and provide measurements of the stellar windparameters. The fits determine Lx/Mdot=2.6+/-0.1 x10^43 erg/s/Msun yr, where Lxis the X-ray luminosity and Mdot is the mass-loss rate of the star. Allowing aninhomogeneous wind improves the fits. IUE spectra show greater P Cygniabsorption during the second half of the orbit than during the first. Wediscuss possible causes of this effect.Comment: 56 pages, 12 figures, to be published in the Astrophysical Journa