Measuring the Mass of the Black Hole in GS2000+25 Using IR Ellipsoidal Variations

Soft X-ray Transients (SXTs) are binary systems that are believed to consist of a black hole and a normal late type dwarf star which fills its Roche Lobe. We have used GRIM II on the ARC 3.5 meter telescope at Apache Point Observatory to obtain infrared photometry of GS2000+25 (QZ Vul). By modeling the SXT ellipsoidal variations with WD98, we can determine the orbital period and inclination of the system. The inclination for a best fit circular orbit is 75 degrees, and when combined with the observed mass function, corresponds to a primary mass of 6.55 solar masses. More data is needed to better define the minima, fill in the small gaps in the light curve, and explore the possibility of an eccentric orbit.