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 Msun black hole (BH) accreting from a 12-15 Msun main sequence star or a 20 Msun BH with a 12-15 Msun giant donor. If the orbital period of the system is ~6 days, a 20 Msun BH model becomes unlikely and we are left with the only possibility that the compact accretor in NGC1313 X-2 is a massive BH of 50-100 Msun. We briefly discuss these results within the framework of an alternative scenario for the formation of ULXs, in which a portion of them may contain BHs of >~30-40 Msun formed from very massive stars in a low metallicity environment.