Constraining the mass of accreting black holes in ultraluminous X-ray sources with ultrafast outflows
Abstract
The nature of ultraluminous X-ray sources (ULXs) - off-nuclear extragalactic sources with luminosity, assumed isotropic, ≳1039 erg s-1 - is still debated. One possibility is that ULXs are stellar black holes (BHs) accreting beyond the Eddington limit. This view has been recently reinforced by the discovery of ultrafast outflows at ∼0.1-0.2c in the high-resolution spectra of a handful of ULXs, as predicted by models of supercritical accretion discs. Under the assumption that ULXs are powered by super-Eddington accretion on to BHs, we use the properties of the observed outflows to self-consistently constrain their masses and accretion rates. We find masses ≲100 M⊙ and typical accretion rates ∼10-5 M⊙ yr-1, I.e. ≈10 times larger than the Eddington limit calculated with a radiative efficiency of 0.1. However, the emitted luminosity is only ≈10 per cent beyond the Eddington luminosity, because most of the energy released in the inner part of the accretion disc is used to accelerate the wind, which implies radiative efficiency ∼0.01. Our results are consistent with a formation model where ULXs are BH remnants of massive stars evolved in low-metallicity environments.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- July 2017
- DOI:
- 10.1093/mnrasl/slx065
- arXiv:
- arXiv:1704.08255
- Bibcode:
- 2017MNRAS.469L..99F
- Keywords:
-
- accretion;
- accretion discs;
- black hole physics;
- binaries: close;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 5 pages, 2 figures, 1 table, accepted for publication in MNRAS Letters