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 non-spherical secondary star, we show that the most likely value for the orbital inclination is 54 +/-1.5 deg. This inclination angle is consistent with those previously published, but has a much smaller error. This inclination implies a primary black hole mass of 6.95 +/-0.6 M_sun. 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 +4/-1.5 deg.