The Spectrum of SMC X-1

The spectrum of SK 160, the optical counterpart of the X-ray source SMC X-1, is discussed and compared with that of the BO.5 Ia standard, HD 152234. Spectra were obtained with the image tube spectrograph on the Mt. Stromlo 2-meter reflector operating at 5000 A/mm with a projected slit of 50 microns to cover the range of 3800 to 5000 A. The absorption lines of SK 160 and HD 152234 are described, and a comparison indicates that SK 160 is an underluminous BO.5 type star, with a mass 30 to 40 times the solar mass. It is noted that the He II 4686 line in the spectrum of SK 160 also appears in other X-ray sources, including 2U 1700-37, Cyg X-1, Cyg X-2, and Cen X-3.