2008ApJ...678.1248V

High-resolution ultraviolet observations of Cygnus X-1 were obtained at two epochs roughly 1 year apart using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Orbital phase ranges around φ_orb=0 and 0.5 were covered at each epoch, and the spectra show P Cygni line features from high (N V, C IV, Si IV) and absorption lines from low (Si II, C II) ionization state material. The high-ionization P Cygni profiles show a marked dependence on orbital phase; strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star. We fit the P Cygni profiles using the Sobolev with exact integration method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. Of the wind-formed lines, the Si IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. The velocity v increases with radius r (normalized to the stellar radius) according to v=v_∞(1-r_*/r)^β, with β~0.75 and v_∞~1420 km/s. Our fit implies a ratio of X-ray luminosity (in units of 10³⁸ erg/s) to wind mass-loss rate (in units of 10^–6 M_☉/yr) of L_X,38/M{dot}_–6~0.033, measured at M{dot}_–6=4.8. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted L_X matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet.