| Summary: | We present a dynamical model of the high mass X-ray binary LMC X-1 based on high-resolution optical spectroscopy and extensive optical and near-infrared photometry. From our new optical data we find an orbital period of P = 3.90917 ± 0.00005 days. We present a refined analysis of the All Sky Monitor data from RXTE and find an X-ray period of P = 3.9094 ± 0.0008 days, which is consistent with the optical period. A simple model of Thomson scattering in the stellar wind can account for the modulation seen in the X-ray light curves. The V – K color of the star (1.17 ± 0.05) implies AV = 2.28 ± 0.06, which is much larger than previously assumed. For the secondary star, we measure a radius of R 2 = 17.0 ± 0.8 R ☉ and a projected rotational velocity of V rotsin i = 129.9 ± 2.2 km s–1. Using these measured properties to constrain the dynamical model, we find an inclination of i = 36fdg38 ± 1fdg92, a secondary star mass of M 2 = 31.79 ± 3.48 M ☉, and a black hole mass of 10.91 ± 1.41 M ☉. The present location of the secondary star in a temperature-luminosity diagram is consistent with that of a star with an initial mass of 35 M ☉ that is 5 Myr past the zero-age main sequence. The star nearly fills its Roche lobe (≈90% or more), and owing to the rapid change in radius with time in its present evolutionary state, it will encounter its Roche lobe and begin rapid and possibly unstable mass transfer on a timescale of a few hundred thousand years.
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