EVIDENCE OF A BLACK HOLE IN THE X-RAY TRANSIENT GS 1354 64 (pBW 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 . The Doppler motions of these features trace an orbit of Å P p days (or its 1 yr alias of days) and a velocity semiamplitude km s 1 (or 2.54448 P p 2.56358 K p 279 5 2 km s ). Both solutions are equally possible. The mass function implied by the shorter period solution K p 292 5 2 is , which, when combined with the rotational broadening of the tidally locked companion f (M) p 5.75 0.30 M, ( km s ), yields a compact object mass of . This is substantially 3 V sin i p 71 4 M sin i p 7.34 0.46 M 1 , 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 ≥27 kpc is inferred by the companion’s luminosity, and this is supported by the large observed systemic velocity ( km s ), which requires such a distance in order to be consistent with the Galactic rotation curve. g p 103 4 Subject headings: accretion, accretion disks — binaries: close — stars: individual (GS 1354 64) — X-rays: stars