Variability of the Hα emission of Cygnus X-1 and its connection with the soft X-ray radiation

High-resolution Hα monitoring of Cyg X-1, HD 226868 was carried out during 1996-2002 and the resultant spectra analysed in conjunction with 1.5-12 keV X-ray monitoring. We demonstrate that the Hα line-profiles have complex variability on different timescales, controlled in particular by the orbital period and the focused wind model of mass loss. We find that long-term variability of the mass loss by the supergiant and short-term variability due to clumpy structure of the stellar wind dominate during the low/hard X-ray state and the X-ray photoionization has a relatively small influence on the line-profile shape and EW variability. During the high/soft X-ray state and flaring the effect of photoionization the line-profile and EW of Hα increases but is still unable to describe the loose anti-correlation between EW and the low energy X-ray emission. We propose that variability of the mass loss by the supergiant can change wind velocities in the Strömgren zone around the accretion disc of the secondary, leading to an increase in accretion rate through the disc.