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⊙ or a 50-100 M⊙ 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⊙ BH accreting from a 12-15 M⊙ main-sequence star or a ∼20 M⊙ BH with a 12-15 M⊙ giant donor. If the modulation of 6.12 ± 0.16 d recently identified as the orbital period of the system is confirmed, a ∼20 M⊙ 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 ∼50-100 M⊙.