Evidence of a black hole in the X-ray transient GS 1354-64 (= BW Circini)

We present the first radial velocity curve of the companion star to BW Cir that demonstrates the presence of a black hole in this X-ray transient that recorded outbursts in 1987 and 1997 (and possibly 1971-1972). We identify weak absorption features corresponding to a G0-5 III donor star, strongly veiled by a residual accretion disk that contributes 61%-65% of the total light at 6300 Angstrom. The Doppler motions of these features trace an orbit of 2.54448 days (or its 1 yr alias P = 2.56358 of days) and a velocity semiamplitude K-2 = 279 +/- 5 km s(-1) (or K-2 = 292 +/- 5 km s(-1)). Both solutions are equally possible. The mass function implied by the shorter period solution is f(M) = 5.75 +/- 0.30 M-circle dot, which, when combined with the rotational broadening of the tidally locked companion (V sin i = 71 +/- 4 km s(-1)), yields a compact object mass of M-1 sin(3) i = 7.34 +/- 0.46 M-circle dot. This is substantially above the mass of a neutron star under any standard equation of state of nuclear matter. The companion star is probably a G subgiant that has evolved off the main sequence in order to fill its Roche lobe. Remarkably, a distance of greater than or equal to27 kpc is inferred by the companion's luminosity, and this is supported by the large observed systemic velocity (gamma = 103 +/- 4 km s(-1)), which requires such a distance in order to be consistent with the Galactic rotation curve.