A DETECTION OF ORBITAL RADIAL-VELOCITY VARIATIONS OF THE PRIMARY COMPONENT IN THE BLACK-HOLE BINARY A0620-00 (= V616 MONOCEROTIS)

Time-resolved spectroscopic observations of Ha emission from the accretion disk in A0620-00 (V616 Mon) are presented. The emission exhibits radial velocity variations at the orbital period; the phase of the variations is shifted by ∼180° from the radial velocity curve of the K star. It is argued that these velocity variations reflect the orbital motion of the compact object. A semiamplitude, K1 = 43 ± 8 km s-1, is derived, which, in conjunction with McClintock and Remillard's value of K2 = 457 ± 8 km s-1, yields a mass ratio, q ( = M1/M2) = 10.6 ± 2.0. Potential systematic errors in our velocity measurements which would lead to increased uncertainty in the mass ratio are discussed. Taken at face value, our determination of the mass ratio results in new lower limits on the masses of the component stars: 3.82 ± 0.24 M⊙ for the compact object, strengthening the case that it is a black hole; and 0.36 ± 0.07 M⊙ for the K star. If the K star has a mass appropriate for a Roche lobe-filling main-sequence star, then the mass of the black hole could be as high as 11 M⊙. © 1990. The American Astronomical Society. All rights.