Program: GS-2009B-Q-20

The masses of the black holes that power ultraluminous X-ray sources (ULXs) have been a topic of strong debate over the last decade, with some evidence pointing to the existence of a new class of intermediate-mass black holes (IMBHs), whilst other evidence suggested the presence of the most extreme accretors amongst stellar mass black holes (albeit black holes possibly ranging in mass up to ~100 M(sun)). Despite a lot of recent attention, this puzzle remains unresolved. Here we propose 10 GMOS-S observations of the optical counterpart to the nearby ULX NGC 1313 X-2, an excellent candidate for radial velocity measurements based on its He II 4686A accretion disc emission line and newly-reported 6-day period. This will allow us to derive the first ever constraints on the mass function of a ULX, and hence the first dynamical mass for a black hole powering a ULX.