Low metallicity natal environments and black hole masses in Ultraluminous X-ray Sources

We review the available estimates of the masses of the compact object in Ultraluminous X-ray Sources (ULXs) and critically reconsider the stellar-mass versus intermediatemass black hole interpretations. Black holes of several hundreds to thousands of M are not required for the majority of ULXs, although they might be present in the handful of known hyper-luminous (∼ 10 erg s−1) objects and/or some sources showing timing features in their power density spectra. At the same time, however, stellar mass BHs may be quite a reasonable explanation for ULXs below ∼ 10 erg s−1, but they need super-Eddington accretion and some suitable dependence of the beaming factor on the accretion rate in order to account for ULXs above this (isotropic) luminosity. We investigate in detail a ’third way’ in which a proportion of ULXs contain ≈ 30−90M black holes formed in a low metallicity environment and accreting in a slightly critical regime and find that it can consistently account for the properties of bright ULXs. Surveys of ULX locations looking for a statistically meaningful relationship between ULX position, average luminosity and local metallicity will provide a definitive test of our proposal.