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 \lambda 6300 .
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 K _ { 2 } = 279 \pm 5 km s ^ { -1 } ( or K _ { 2 } = 292 \pm 5 km s ^ { -1 } ) .
Both solutions are equally possible .
The mass function implied by the shorter period solution is f ( M ) = 5.75 \pm 0.30 M _ { \odot } which , combined with the rotational broadening of the tidally locked companion ( V \sin i = 71 \pm 4 km s ^ { -1 } ) , yields a compact object mass of M _ { 1 } \sin ^ { 3 } i = 7.34 \pm 0.46 M _ { \odot } .
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 \geq 27 kpc is inferred by the companion ’ s luminosity and this is supported by the large observed systemic velocity ( \gamma = 103 \pm 4 km s ^ { -1 } ) which requires such a distance in order to be consistent with the Galactic rotation curve .