Low-metallicity natal environments and black hole masses in ultraluminous X-ray sources
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 (BH) interpretations. BHs of several hundreds to thousands of Msolar are not required for the majority of ULXs, although they might be present in the handful of known hyperluminous (~1041 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 ~1040 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 ~30-90Msolar 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- December 2009
- DOI:
- 10.1111/j.1365-2966.2009.15509.x
- arXiv:
- arXiv:0909.1017
- Bibcode:
- 2009MNRAS.400..677Z
- Keywords:
-
- accretion;
- accretion discs;
- black hole physics;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 11 pages, 1 figure, accepted by MNRAS