CYG X-1 = V1357 CYG investigation based on high-resolution optical spectroscopy of 2002–2004

We report the results of Cyg X-1 spectral monitoring for 2002–2004. A comparison of observed and non-local thermal equilibrium model profiles for H I, He I and Mg II is given, taking into account the tidal distortion of the Cyg X-1 optical component and its illumination by X-ray emission from the secondary component. We set limits on the main characteristics of the optical component, T eff=30 400±500K and log g=3.31±0.07, and on the overabundances of He and Mg, [He]/[H]=0.43±0.06 dex and [Mg]/[H]=0.75±0.15 dex. The Doppler images were reconstructed by an improved Doppler tomography method on the basis of nine He II (λ=4686 A) profiles from 2003 (the ‘soft’ X-ray state) and six profiles from 2004 (the ‘hard’ X-ray state). This allowed us to set limits on the black-hole-to-supergiant mass ratio (1/4)≤slant M X/M O≤slant (1/3).