EVIDENCE OF A BLACK HOLE IN THE X-RAY TRANSIENT GS 1354-64 ( = BW CIRCINI)

摘要

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 A. 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 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 solar mass, 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 solar mass. This is substantially above the mass of a neutron star under any standard equation of state of nuclear mailer. 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 (γ = 103 +- 4 km s~(-1)), which requires such a distance in order to be consistent with the Galactic rotation curve.