The Mass Ratio and the Disk Image of the X-Ray Nova GS 2000+25

Keck observations of the black hole candidate GS 2000+25 in quiescence have recently been used by Filippenko, Matheson, and Barth to determine a mass function f(Mx) = 4.97 ± 0.10 solar mass for the compact object. Our reanalysis of the data confirms this result (5.01 ± 0.12 solar mass). We estimate a mass ratio of q = Mc/Mx = 0.042 ± 0.012 from the rotational broadening of the companion star, v sin i = 86 ± 8 km s-1. From q and the companion star's radial velocity Kc, we derive the mass of the compact object Mx = (5.44 ± 0.15) sin-3 i solar mass and the mass of the companion star Mc = (0.23 ± 0.02) sin-3 i solar mass. Constraints on the inclination (75 degrees > i > 47 degrees) lead to 6.04 < Mx < 13.9 and 0.26 < Mc < 0.59 (1-sigma) in solar mass units. We determine a spectral type of K3-K6 for the companion star, which contributes more than 72% of the light at red wavelengths (94% ± 5% for K5 V). Our analysis shows that the companion is an undermassive star, slightly evolved but not a subgiant. Consistent with the work of Filippenko et al. we detect Li I lambda-6708 absorption (equivalent width 150 ± 85 mA) in the spectrum of the companion. A Doppler image of the system shows evidence for a bright spot which contributes 10% of the emission-line flux density, and arises in the range 0.3-0.6 RL1 of the accretion disk, where L1 is the inner Lagrangian point. Along the trajectory of the gas stream, the velocities are initially ballistic and gradually become Keplerian.