Program: GN-2006B-Q-39

The compact X-ray source ``X-7'' in M33 is the first eclipsing black hole binary to be discovered. The proposed GMOS observations -- plus the extraordinary eclipse and distance constraints that are not available for Galactic black-hole binaries -- will allow us to obtain a black hole mass measurement of unprecedented precision, namely, about 5\%. Further, we will use the black hole mass and the X-ray continuum data from seven deep Chandra observations to estimate the spin of X-7. Obtaining the defining properties of mass and spin for this unique black hole at 795 kpc will mark a major advance in our capability to study black holes. Future X-ray eclipse-mapping studies of X-7 can be expected to provide unique information on conditions near the event horizon of a black hole. Furthermore, X-7 would be one of only a few black holes holes to have its spin estimated, and the measurement of spin is central to models of relativistic black-hole jets, models of core-collapse SNe, and to future gravitational-wave studies of merging black holes.