Tomographic analysis of Hα profiles in HDE 226868/CYGNUS X-1

We present high-quality H alpha 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 lambda 6678 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 H alpha 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 phi(0) = 0.86; these parameters are similar to those for the He II lambda 4686 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 H alpha emission from the accretion disk surrounding the companion (any such emission contributes less than 3% of the continuum intensity). The results demonstrate that the H alpha line provides an important probe of the mass transfer process that eventually fuels the X-ray source.