1999PASP..111..969F

We have obtained 17 moderate-resolution (∼2.5 Å) optical spectra of the Galactic X-ray Nova Velorum 1993 in quiescence with the Keck II telescope. Cross-correlation with the spectra of late-type dwarfs (especially K7-M0) yields excellent radial velocities for the secondary star. The orbital period (P) is 0.285206±0.0000014 days, and the semiamplitude (K₂) is 475.4±5.9 km.s^–1. Our derived mass function, f(M₁)=PK³₂/2πG=3.17±0.12 M_☉, is close to the conventional absolute limiting mass for a neutron star (∼3.0-3.2 M_☉)–but if the orbital inclination i≲80° (given the absences of eclipses), then M₁≳4.2-4.4 M_☉ for nominal secondary-star masses of 0.5 M_☉ (M0) to 0.65 M_☉ (K6). Even if the secondary is quite undermassive (e.g., M₂=0.3 M_☉), we derive M₁≳3.9 M_☉. The primary star is therefore almost certainly a black hole rather than a neutron star.

Fits to the wings of the double-peaked Hα emission line yield approximate radial velocities for the compact primary. The velocity curve has a semiamplitude (K₁) of 65.3±7.0 km.s^–1, but with a phase offset by 237° (rather than 180°) from that of the secondary star. Under the assumption that the observed semiamplitude reflects the motion of the primary (despite this offset, which is common), the mass ratio q=M₂/M₁=K₁/K₂=0.137±0.015, and hence for M₂=0.5-0.65 M_☉ we derive M₁=3.64-4.74 M_☉. The constraints from q and the mass function yield M₁=4.4 M_☉ and i~78° if we use a normal K7-K8 secondary (M₂~0.6 M_☉). Indeed, consistency cannot be achieved for M₂≲0.59 M_☉ if the maximum inclination estimate (80°) is correct. An adopted mass M₁~4.4 M_☉ is significantly below the typical value of ∼7 M_☉ found for black holes in other low-mass X-ray binaries.

Keck observations of MXB 1659-29 (V2134 Oph) in quiescence reveal a probable optical counterpart at R=23.6±0.4 mag. This object recently went into a new X-ray and optical (R~18 mag) outburst, after about 21 years of inactivity.