EVIDENCE FOR A BLACK-HOLE IN THE X-RAY BINARY NOVA MUSCAE-1991

Optical photometry and spectroscopy of the X-ray Nova Muscae 1991 in quiescence reveal an orbital period of 10.398 +/- 0.014 hr and an absorption-line velocity curve consistent with a sinusoidal modulation at a half-amplitude of 409 +/- 18 km s-1. The spectral type of the secondary star is in the range K0 V to K4 V. The value of the mass function, 3.1 +/- 0.4 MD. is a conservative lower limit on the mass of the compact primary and suggests that the primary is a black hole. Further considerations of the binary inclination angle and the mass of the secondary strengthen the black hole model. The folded light curves in the I band and the B + V band resemble ellipsoidal variations, with an additional brightening near one of the maxima in the B + V band. The orbital period is 1.4% shorter than the photometric period observed during outburst, as expected if the outburst modulations are analogs of "superhumps" in dwarf novae. In quiescence, the optical properties of the X-ray binary Nova Muscae 1991 bear a striking resemblance to the black hole binary A0620-00 (Nova Mon 1917, 1975), which extends the basis of similarity that was demonstrated during outburst at X-ray and optical wavelengths.