The black hole in NGC 1313 X-2: constraints on the mass from optical observations

We present a theoretical study on the nature of the ultraluminous X-ray source NGC 1313 X-2. We evolved a set of binaries with high-mass donor stars orbiting a 20 M(circle dot) or a 50-100 M(circle dot) black hole (BH). Using constraints from optical observations, we restricted the candidate binary system for NGC 1313 X-2 to be either a 50-100 M(circle dot) BH accreting from a 12-15 M(circle dot) main-sequence star or a similar to 20 M(circle dot) BH with a 12-15 M(circle dot) giant donor. If the modulation of 6.12 +/- 0.16 d recently identified as the orbital period of the system is confirmed, a similar to 20 M(circle dot) BH model becomes unlikely and we are left with the only possibility that the compact accretor in NGC 1313 X-2 is a massive BH of similar to 50-100 M(circle dot).