A Black Hole in the X-Ray Nova Velorum 1993
Abstract
We have obtained 17 moderate-resolution (~2.5 A) optical spectra of the Galactic X-ray Nova Velorum 1993 in quiescence with the Keck-II telescope. The orbital period (P) is 0.285206 +/- 0.0000014 d, and the semiamplitude (K2) is 475.4 +/- 5.9 km/s. Our derived mass function, f(M1) = PK23 /2 pi G = 3.17 +/- 0.12 Msun, is close to the conventional absolute limiting mass for a neutron star (~ 3.0-3.2 Msun) -- but if the orbital inclination i is less than 80 degrees (given the absences of eclipses), then M1 is greater than 4.2-4.4 Msun for nominal secondary-star masses of 0.5 Msun (M0) to 0.65 Msun (K6). The primary star is therefore almost certainly a black hole rather than a neutron star. The velocity curve of the primary from H-alpha emission has a semiamplitude (K1) of 65.3 +/- 7.0 km/s, but with a phase offset by 237 degrees (rather than 180 degrees) from that of the secondary star. The nominal mass ratio q = M2/M1 = K1/K2 = 0.137 +/- 0.015, and hence for M2 = 0.5-0.65 Msun we derive M1 = 3.64-4.74 Msun. An adopted mass M1 ~ 4.4 Msun is significantly below the typical value of ~ 7 Msun 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.
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