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arXiv · 0909.1017

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

L. Zampieri·T. P. Roberts
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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.

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L. Zampieri, T. P. Roberts. 2009-09-05. Low metallicity natal environments and black hole masses in Ultraluminous X-ray Sources. https://doi.org/10.1111/j.1365-2966.2009.15509.x

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