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 HST light curves , the orbital period , and the black hole ’ s radial velocity amplitude based on the He II \lambda 4686 Å disk emission line shift of \sim 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 Myrs 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 _ { \odot } that can be produced from stellar evolution of low metalicity stars , or an IMBH of a few hundred to above 1000 M _ { \odot } if its true radial velocity 2 K ^ { \prime } < 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 black hole or an IMBH .