Extragalactic X-ray binary systems - I. SMC X-1 ≡ Sk 160.

Properties of the extragalactic binary X-ray system SMC X-1/Sk 160 are analyzed using constraints imposed by X-ray and optical data, distance, X-ray heating effects, and membership in the Small Magellanic Cloud. Three different optical light curves of Sk 160 are solved, and the existence of multiple sets of solutions is demonstrated. It is shown that variable X-ray heating alone can account for the different observed optical light curves if allowance is made for observational difficulties and circumstellar absorption. Analysis of the X-ray heating effects indicates that the X-ray intensity in the direction of earth may not be the same as the intensity in the direction of Sk 160. Due to large uncertainties in the system parameters, ranges of values are derived for the masses of the two components, the mass ratio, the angle between the normal to the orbital plane and the line of sight, the radial velocity of the system, and its X-ray luminosity.