OPTICAL STUDIES OF V404 CYG, THE X-RAY TRANSIENT GS-2023+338 .4. THE ROTATION SPEED OF THE COMPANION STAR

We have resolved the rotationally broadened absorption features of the secondary star in the black hole X-ray transient GS 2023 + 338 (= V404 Cyg). Using template stars that cover spectral types G8-K2 and luminosity classes III to V, we derive a rotation speed of V(rot) sin i = 39.1 +/- 1.2 km s-1. This implies a mass ratio of q = M(c)/M(x) = 0.060-0.005(+0.004) for the case of a tidally locked and Roche-lobe-filling secondary. Furthermore, the updated radial velocity curve of the companion star permits us to refine the mass function to 6.08 +/- 0.06 M., thus confirming and strengthening the case for a black hole primary in V404 Cyg. An updated version of the orbital parameters is also presented.