Evidence for a Black Hole in the X-ray Transient Gs 1354-64 (=bw Cir)

We present the first radial velocity curve of the companion star to BW Cir which demonstrates the presence of a black hole in this X-ray transient which recorded outbursts in 1987 and 1997 (and possibly 1971-2). We identify weak absorption features corresponding to a G0-5III donor star, strongly veiled by a residual accretion disc which contributes 61-65 % of the total light at λ6300. The Doppler motions of these features trace an orbit of P = 2.54448 days (or its 1-yr alias of P = 2.56358 days) and velocity semi-amplitude K2 = 279 ± 5 km s −1 (or K2 = 292 ± 5 km s ). Both solutions are equally possible. The mass function implied by the shorter period solution is f(M) = 5.75± 0.30M⊙ which, combined with the rotational broadening of the tidally locked companion (V sin i = 71 ± 4 km s), yields a compact object mass of M1 sin 3 i = 7.34 ± 0.46M⊙. 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 which has evolved off the main sequence in order to fill its Roche lobe. Remarkably, a distance of ≥27 kpc is inferred by the companion’s luminosity and this is supported by the large observed systemic velocity (γ = 103± 4 km s) which requires such a distance in order to be consistent with the Galactic rotation curve. Subject headings: accretion, accretion disks binaries: close stars: individual: BW Cir (=GS 1354-64) X-rays:stars