Ultraluminous X-ray Sources forming in low metallicity natal environments

TL;DR

This paper explores the formation of ultraluminous X-ray sources (ULXs) in low metallicity environments, proposing that they originate from massive stellar black holes formed in such conditions, explaining their high luminosities.

Contribution

It introduces a model linking low metallicity environments to the formation of massive stellar black holes that power ULXs, supported by observational and evolutionary calculations.

Findings

Massive black holes (30-90 M_\u2297) can form in low metallicity environments.

Over 10^5 such black holes may have formed in the Cartwheel galaxy.

Binary evolution suggests some ULXs contain black holes of 50-100 M_rom massive donors.

Abstract

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 $\sim 30-90 M_\odot$ 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 $50-100 M_\odot$ black hole.