The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1

TL;DR

This study uses high-resolution ultraviolet and optical spectroscopy to refine the physical parameters of the supergiant in Cyg X-1, leading to updated mass estimates for both the star and its black hole companion.

Contribution

It provides new non-LTE model-based measurements of the supergiant's temperature, gravity, and chemical abundances, and derives the masses of the binary components considering multiple observational constraints.

Findings

Supergiant's temperature is 28,000 K with lower gravity than previous estimates.

Mass of the supergiant is approximately 23 solar masses.

Black hole's mass is approximately 11 solar masses.

Abstract

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 +/- 2.5kK and log g > 3.00 +/- 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_sun for the supergiant and 11^{+5}_{-3} M_sun for the black hole.