The black hole in NGC 1313 X-2

The amount of data available for NGC 1313 X-2 make it a cornerstone for the study of ultraluminous X-ray sources (ULXs). We modelled the optical and X-ray data of this ULX with a binary evolution code that takes into account X-ray irradiation. We restricted the candidate binary system to be either a 50-100 M⊙ black hole (BH) accreting from a 12-15 M⊙ main sequence star or a 20 M⊙ BH with a 12-15 M⊙ giant donor. If the orbital period of the system is ∼6 days, 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⊙. We briefly discuss these results within the framework of an alternative scenario for the formation ofULXs, in which a portion ofthem may contain BHs of ≳30-40 M⊙ formed from very massive stars in a low metallicity environment