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 o f the now quiescent Galactic X-ray nova GRO J0422 + 32, obtained in 19 94 November and 1995 January with the W. M. Keck 10 m telescope, is us ed to derive the physical parameters of the binary system. The H alpha emission-line profile exhibits large variations in consecutive half-h our exposures taken in 1994 November, but smaller variations in 1995 J anuary. Crosscorrelation of the 6000 - 6500 Angstrom spectral region w ith that of late-type dwarf stars yields reliable absorption-line radi al velocities for the secondary star. The orbital period is found to b e 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 secondar y 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 approxim ate 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 addit ional emission components on the accretion disk hence, the observed se mi-amplitude does not necessarily reflect the true motion of the compa ct 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 sugges ted by its spectrum and (independently) by the requirement that it fil l its Roche lobe, the mass of the primary is 3.57 +/- 0.34 M., somewha t higher than the theoretical upper limit (similar to 3.2 M.) for a sl owly rotating neutron star with an extremely stiff equation of state, and considerably above the measured masses of neutron stars. We conclu de that the compact object is probably a black hole, as suggested by i ts hard X-ray spectrum during outburst. The derived inclination angle of the system (48 +/- 3 degrees) is consistent with the apparent absen ce of eclipses of the accretion disk.