2012ApJ...745...89L

Dynamical mass measurements hold the key to answering whether ultraluminous X-ray sources (ULXs) are intermediate-mass black holes (IMBHs) or stellar-mass black holes with special radiation mechanisms. NGC 1313 X-2 is so far the only ULX with Hubble Space Telescope light curves, the orbital period, and the black hole's radial velocity amplitude based on the He II λ4686 disk emission line shift of ∼200 km/s. We constrain its black hole mass and other parameters by fitting observations to a binary light curve code with accommodations for X-ray heating of the accretion disk and the secondary. Given the dynamical constraints from the observed light curves and the black hole radial motion and the observed stellar environment age, the only acceptable models are those with 40-50 Myr old intermediate-mass secondaries in their helium core and hydrogen shell burning phase filling 40%-80% of their Roche lobes. The black hole can be a massive black hole of a few tens of M_☉ that can be produced from stellar evolution of low-metallicity stars, or an IMBH of a few hundred to above 1000 M_☉ if its true radial velocity 2K' < 40 km/s. Further observations are required to better measure the black hole radial motion and the light curves in order to determine whether NGC 1313 X-2 is a stellar-mass black hole or an IMBH.