A black hole in the X-ray Nova Ophiuchi 1977

We have obtained 18 moderate-resolution (similar to 2.5 Angstrom) spectra of the Galactic X-ray Nova Ophiuchi 1977 in quiescence with the first of the W. M. Keck 10-m telescopes. Cross correlation with the spectra of late-type dwarfs (especially K7) yields excellent radial velocities for the companion star. The orbital period (P) is 0.5229 +/- 0.0044 d, and the semiamplitude (K-c) is 447.6 +/- 3.9 km s(-1); these are consistent with the values previously derived by Remillard et al. [ApJ, 459, 226 (1996)] from CTlO photometry (P = 0.5213 +/- 0.0013 d) and spectroscopy (K-c = 420 +/- 30 km s(-1)). Our derived mass function, f(M-x) = PKc3/2 pi G, is 4.86 +/- 0.13 M-. (cf. 4.0 +/- 0.8, Remillard et al.), the third highest known for any low-mass X-ray binary. Being considerably above the conventional limiting mass for a neutron star (similar to 3.2 M-.), it strongly suggests that the compact primary is a black hole. Plausible mass estimates based on an inclination i = 70 degrees (from ellipsoidal modulations) and M-c = 0.3-0.6 M-. are M-x = 6.4-6.9 M-..