2009MNRAS.400..677Z

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 intermediate-mass black hole (BH) interpretations. BHs 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 hyperluminous ( ∼ 10⁴¹ erg/s) 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 ∼ 10⁴⁰ erg/s, 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_☉ BHs 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.