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[sun]) stars in low metallicity natal environments. Such black holes have a typical mass in the range ~30-90 M[sun] and may account for the properties of bright (above ~10(40) erg s−1) ULXs. More than ~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 50-100 M[sun] black hole.