The mass donor of Scorpius X-1 revealed

We present the first detection of the mass donor star in the prototype X-ray binary Scorpius X-1. Phase resolved spectroscopy revealed narrow emission lines components from the irradiated secondary star. Radial velocity fits to the Bowen blend emission are used to establish an absolute orbital ephemeris and derive an accurate value for the systemic velocity; gamma= -113.8 +/- 0.6 km/s. Emission from the irradiated front side of the secondary leads to solid limits to the radial velocity of the mass donor of 87 < K_2 < 148 km/s. In conjunction with an upper limit to the velocity of the primary K_1 < 53 km/s, we derive a firm limit on the mass ratio of Sco X-1 of q < 0.61. % A likely set of system parameters satisfying the various constraints as well as the recent inclination estimate by Fomalont et al. (2001) is K_1=40 km/s, K_2=133 km/s, q=0.30 corresponding to component masses of M_1=1.4M_sun, M_2=0.42 M_sun for an orbital inclination of 38 degrees.