CAL 87 - An eclipsing black hole binary?

Simultaneous spectroscopic and photometric observations have been made of the optical counterpart of the variable X-ray source CAL 87, an apparent low-mass X-ray binary in the Large Magellanic Cloud. The shape of the optical light curve, showing a deep minimum with very broad wings, suggests an eclipse of a very extended disk structure, which is the dominant light source. The only strong feature in the optical spectrum is a variable He II (4686 A) emission line. Radial velocities of the He II emission lines show a small-amplitude variation (K about 40 km/s) through the orbital period. The phasing of these velocities indicates that the emission arises on the degenerate star side of the center of mass, as is the case for many low-mass X-ray binaries. If this velocity reflects the motion of the compact star, then the mass ratio must be large (about 10), and there is a strong probability that the system may be an eclipsing black hole binary. 21 refs