THE MASS OF THE PROBABLE BLACK-HOLE IN THE X-RAY NOVA GRO J0422+32

A series of 21 moderate-resolution (similar to 2.4 Angstrom) spectra of the now quiescent Galactic X-ray nova GRO J0422 + 32, obtained in 1994 November and 1995 January with the W. M. Keck 10 m telescope, is used to derive the physical parameters of the binary system. The H alpha emission-line profile exhibits large variations in consecutive half-hour exposures taken in 1994 November, but smaller variations in 1995 January. Crosscorrelation of the 6000 - 6500 Angstrom spectral region with that of late-type dwarf stars yields reliable absorption-line radial velocities for the secondary star. The orbital period is found to be O-d.21159 +/- 0.(d)00057, with a semiamplitude of 380.6 +/- 6.5 km s(-1); the implied mass function is 1.21 +/- 0.06 M.. Inspection of the averaged spectrum of GRO J0422 + 32 in the rest frame of the secondary star suggests that the secondary is an M2 V star, but the accretion disk contributes 30% - 60% of the light at similar to 6300 A. Fits to the wings of the strong, double-peaked Ha emission line yield approximate radial velocities for the compact primary; the velocity curve has a semi-amplitude of 41.6 +/- 3.2 km s(-1), but with a phase offset by 253 degrees (rather than 180 degrees) from that of the secondary star. The offset, which is similar to that of several other X-ray novae and many dwarf novae, may be indicative of geometric distortions or additional emission components on the accretion disk hence, the observed semi-amplitude does not necessarily reflect the true motion of the compact primary. Under the assumption that it does, however, we find q = 0.1093 +/- 0.0086, the mass ratio of the secondary to the primary. If the secondary star is a normal M2 dwarf (M = 0.39 +/- 0.02 M.), as suggested by its spectrum and (independently) by the requirement that it fill its Roche lobe, the mass of the primary is 3.57 +/- 0.34 M., somewhat higher than the theoretical upper limit (similar to 3.2 M.) for a slowly rotating neutron star with an extremely stiff equation of state, and considerably above the measured masses of neutron stars. We conclude that the compact object is probably a black hole, as suggested by its hard X-ray spectrum during outburst. The derived inclination angle of the system (48 +/- 3 degrees) is consistent with the apparent absence of eclipses of the accretion disk.