Tomographic Analysis of Hα Profiles in HDE 226868/Cygnus X-1

We present high-quality Hα spectra of the massive X-ray binary HDE 226868 = Cyg X-1 that were made in 1985 and 1986. We measured radial velocities using the He I λ6678 line, and we use the resulting orbital solution together with a Hipparcos light curve of the ellipsoidal variation to arrive at a revised period estimate of P = 5.59977 +/- 0.00002 days. We analyze the orbital phase-related variations in Hα emission using a Doppler tomography algorithm, and we show that the profile variations are well matched by a linear combination of two components: a P Cygni profile that moves with the motion of the supergiant star and an emission component that follows an antiphase but phase-shifted velocity curve (semiamplitude K_em = 68 km s^-1 and radial velocity maximum at φ₀ = 0.86; these parameters are similar to those for the He II λ4686 emission line). We use a vector decomposition of this motion to argue that the latter emission component forms between the stars in a focused wind flow from the supergiant to the unseen companion. We searched for but found no clear evidence of Hα emission from the accretion disk surrounding the companion (any such emission contributes less than 3% of the continuum intensity). The results demonstrate that the Hα line provides an important probe of the mass transfer process that eventually fuels the X-ray source.