Dynamical Constraints on the Black Hole in Ultraluminous X-ray Source NGC 1313 X-2

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. NGC1313 X-2 is so far the only ULX with HST light curves, the orbital period, and the black hole's radial velocity amplitude. 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, the presence of hydrogen lines in the optical spectra, 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 solar masses that can be produced from stellar evolution of low metalicity stars, or an IMBH of a few hundred to above 1000 solar masses if its true radial velocity is 5 times lower than derived from the He II disk emission line. Further observations with HST can better measure the black hole radial motion and the light curves and determine whether NGC1313 X-2 is a stellar black hole or an IMBH.