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

TL;DR

This paper reviews black hole mass estimates in ULXs, arguing most do not require intermediate-mass black holes, and proposes low-metallicity stellar-mass black holes as a plausible explanation for many ULXs.

Contribution

It introduces a 'third way' hypothesis that low-metallicity environments produce black holes of 30-90 solar masses, explaining bright ULXs without invoking intermediate-mass black holes.

Findings

Most ULXs do not require intermediate-mass black holes.

Stellar-mass black holes with super-Eddington accretion can explain ULXs below 10^{40} erg s^{-1}.

Low-metallicity environments may produce 30-90 M_d black holes accounting for bright ULXs.

Abstract

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 interpretations. Black holes of several hundreds to thousands of $M_\odot$ are not required for the majority of ULXs, although they might be present in the handful of known hyper-luminous ($\sim 10^{41}$ 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 $\sim 10^{40}$ 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 $\approx 30-90 M_\odot$ 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.