Observations of the X-ray nova GRO J0422+32. II. Optical spectra approaching quiescence

We present results obtained from a series of 5 Å resolution spectra of X-ray nova GRO J0422 + 32 obtained in 1993 October, when the system was approximately 2 mag above quiescence, with R ∼ 19. The data were obtained in an effort to measure the orbital radial velocity curve of the secondary, but detection of the narrow photospheric absorption lines needed to do this proved elusive. Instead we found wide absorption bands reminiscent of M star photospheric features. The parameters determined by fitting accretion disk line profiles (Smak profiles) to the Hα line are similar to those found in several strong black hole candidates. Measurements of the velocity of the Hα lines are consistent with an orbital period of 5.1 hr and a velocity semiamplitude of the primary of 34 ± 6 km s^-1. These measurements, when combined with measurements of the velocity semiamplitude of the secondary made by others, indicate that the mass ratio q ∼ 0.09. If the secondary follows the empirical mass-radius relation found for cataclysmic variables, the low q implies a primary mass of M^X ∼ 5.6 M_⊙, and a rather low (face-on) inclination. The Hα EW is found to be modulated on the orbital period with a phasing that implies a partial eclipse of the disk by the secondary, but simultaneous R-band photometry shows no evidence for such an eclipse.