Ultraluminous X-ray Sources forming in low metallicity natal environments

In the last few years multiwavelength observations have boosted our understanding of Ultraluminous X-ray Sources (ULXs). Yet, the most fundamental questions on ULXs still remain to be definitively answered: do they contain stellar or intermediate mass black holes? How do they form? We investigate the possibility that the black holes hosted in ULXs originate from massive (40-120 $M_{\odot }$) stars in low metallicity natal environments. Such black holes have a typical mass in the range $\sim 30-90M_{\odot }$ and may account for the properties of bright (above $\sim {10}^{40}$ erg s${}^{-1}$) ULXs. More than $\sim 10^5$ massive black holes might have been generated in this way in the metal poor Cartwheel galaxy during the last ${10}^7$ years and might power most of the ULXs observed in it. Support to our interpretation comes from NGC 1313 X-2, the first ULX with a tentative identification of the orbital period in the optical band, for which binary evolution calculations show that the system is most likely made by a massive donor dumping matter on a $50-100M_{\odot }$ black hole.