Cristina Zurita

SWIFT J1753.5-0127 : THE BLACK HOLE CANDIDATE WITH THE SHORTEST ORBITAL PERIOD

We present time-resolved photometry of the optical counterpart to the black hole candidate Swift J1753.5–0127 which has remained in the low/hard X-ray state and bright at optical/IR wavelengths since its discovery in 2005. At the time of our observations Swift J1753.5–0127 does not show a decay trend but remains stable at R = 16.45 with a night-to-night variability of ~0.05 mag. The R-band light curves, taken from 2007 June 3 to August 31, are not sinusoidal, but exhibit a complex morphology with remarkable changes in shape and amplitude. The best period determination is 3.2443 ± 0.0010 hr. This photometric period is likely a superhump period, slightly larger than the orbital period. Therefore, Swift J1753.5–0127 is the black hole candidate with the shortest orbital period observed to date. Our estimation of the distance is comparable to values previously published and likely places Swift J1753.5–0127 in the Galactic halo.

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 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 K2 = 279 ± 5 km s-1 (or K2 = 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☉, 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 M1 i = 7.34 ± 0.46 M☉. 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 ≥27 kpc is inferred by the companions 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.