We present an examination of high resolution , ultraviolet spectroscopy from Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the massive X-ray binary HD 226868 = Cyg X-1 .
We analyzed this and ground-based optical spectra to determine the effective temperature and gravity of the O9.7 Iab supergiant .
Using non-local thermodynamic equilibrium ( non-LTE ) , line blanketed , plane parallel models from the TLUSTY grid , we obtain T _ { eff } = 28.0 \pm 2.5 kK and \log g \gtrsim 3.00 \pm 0.25 , both lower than in previous studies .
The optical spectrum is best fit with models that have enriched He and N abundances .
We fit the model spectral energy distribution for this temperature and gravity to the UV , optical , and IR fluxes to determine the angular size of and extinction towards the binary .
The angular size then yields relations for the stellar radius and luminosity as a function of distance .
By assuming that the supergiant rotates synchronously with the orbit , we can use the radius – distance relation to find mass estimates for both the supergiant and black hole as a function of the distance and the ratio of stellar to Roche radius .
Fits of the orbital light curve yield an additional constraint that limits the solutions in the mass plane .
Our results indicate masses of 23 ^ { +8 } _ { -6 } M _ { \odot } for the supergiant and 11 ^ { +5 } _ { -3 } M _ { \odot } for the black hole .