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. Photometric observations show a pronounced light variation (> 1 mag) with a period of 10.6 hr. 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. There appears to be little X-ray heating of the nondegenerate companion, possibly due to the opacity of the thick disk. The only strong feature in the optical spectrum is a variable He II (λ4686) emission line. Near minimum light, weak absorption lines (e.g., H and K of Ca II, G band) presumably from the nondegenerate companion are visible, indicating a late-F spectral type. Radial velocities of the He II emission lines show a small-amplitude variation (K ~ 40 km s^-1^) 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 (~ 10), and there is a strong probability that the system may be an eclipsing black hole binary.