Infrared Observations of Nova Muscae 1991: Black Hole Mass Determination from Ellipsoidal Variations

We have obtained infrared photometry for the soft X-ray transient GU Mus. We present J - and K s -band light curves modeled with WD98, the newest version of the Wilson-Devinney light curve–modeling code. Using detailed models for the expected ellipsoidal variations due to the nonspherical secondary star, we show that the most likely value for the orbital inclination is 54° ± 1 5. This inclination angle is consistent with those previously published, but it has a much smaller error. This inclination implies a primary black hole mass of 6.95 ± 0.6 M ⊙ . While we do not see any evidence for contamination of our infrared light curves from other sources in the system, a conservative model with a contamination level of 15% increases the uncertainty in the inclination angle to 54 deg.