A Black Hole in the X-Ray Nova Ophiuchi 1997

We have obtained 18 moderate-resolution (~2.5 Angstroms) 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 (Kc) is 447.6 ± 3.9 km s-1; these are consistent with the values previously derived by Remillard et al. (1996) from CTIO photometry (P = 0.5213 ± 0.0013 d) and spectroscopy (Kc = 420 ± 30 km s-1). Our derived mass function, f(Mx) = PKc3/2 pi G, is 4.86 ± 0.13 Modot (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 (~3.2 Modot), it strongly suggests that the compact primary is a black hole. Plausible mass estimates based on an inclination i = 70circ (from ellipsoidal modulations) and Mc = 0.3-0.6 Modot are Mx = 6.4-6.9 Modot.