We present the results of a spectroscopic monitoring program ( from 1998 to 2002 ) of the H \alpha emission strength in HDE 226868 , the optical counterpart of the black hole binary , Cyg X-1 .
The feature provides an important probe of the mass loss rate in the base of the stellar wind of the supergiant star .
We derive an updated ephemeris for the orbit based upon radial velocities measured from He i \lambda 6678 .
We list net equivalent widths for the entire H \alpha emission/absorption complex , and we find that there are large variations in emission strength over both long ( years ) and short ( hours to days ) time spans .
There are coherent orbital phase related variations in the profiles when the spectra are grouped by H \alpha equivalent width .
The profiles consist of ( 1 ) a P Cygni component associated with the wind of the supergiant , ( 2 ) emission components that attain high velocity at the conjunctions and that probably form in enhanced outflows both towards and away from the black hole , and ( 3 ) an emission component that moves in anti-phase with the supergiant ’ s motion .
We argue that the third component forms in accreted gas near the black hole , and the radial velocity curve of the emission is consistent with a mass ratio of M _ { X } / M _ { opt } \approx 0.36 \pm 0.05 .
We find that there is a general anti-correlation between the H \alpha emission strength and X-ray flux ( from the Rossi X-ray Timing Explorer All Sky Monitor instrument ) in the sense that when the H \alpha emission is strong ( W _ { \lambda } < -0.5 Å ) the X-ray flux is weaker and the spectrum harder .
On the other hand , there is no correlation between H \alpha emission strength and X-ray flux when H \alpha is weak .
We argue that this relationship is not caused by wind X-ray absorption nor by the reduction in H \alpha emissivity by X-ray heating .
Instead , we suggest that the H \alpha variations track changes in wind density and strength near the photosphere .
The density of the wind determines the size of X-ray ionization zones surrounding the black hole , and these in turn control the acceleration of the wind in the direction of the black hole .
During the low/hard X-ray state , the strong wind is fast and the accretion rate is relatively low , while in the high/soft state the weaker , highly ionized wind attains only a moderate velocity and the accretion rate increases .
We argue that the X-ray transitions from the normal low/hard to the rare high/soft state are triggered by episodes of decreased mass loss rate in the supergiant donor star .