Efficiency of super-Eddington magnetically-arrested accretion
Abstract
The radiative efficiency of super-Eddington accreting black holes (BHs) is explored for magnetically-arrested discs, where magnetic flux builds-up to saturation near the BH. Our three-dimensional general relativistic radiation magnetohydrodynamic (GRRMHD) simulation of a spinning BH (spin a/M = 0.8) accreting at ∼50 times Eddington shows a total efficiency ∼50 per cent when time-averaged and total efficiency ≳ 100 per cent in moments. Magnetic compression by the magnetic flux near the rotating BH leads to a thin disc, whose radiation escapes via advection by a magnetized wind and via transport through a low-density channel created by a Blandford-Znajek (BZ) jet. The BZ efficiency is sub-optimal due to inertial loading of field lines by optically thick radiation, leading to BZ efficiency ∼40 per cent on the horizon and BZ efficiency ∼5 per cent by r ∼ 400rg (gravitational radii) via absorption by the wind. Importantly, radiation escapes at r ∼ 400rg with efficiency η ≈ 15 per cent (luminosity L ∼ 50LEdd), similar to η ≈ 12 per cent for a Novikov-Thorne thin disc and beyond η ≲ 1 per cent seen in prior GRRMHD simulations or slim disc theory. Our simulations show how BH spin, magnetic field, and jet mass-loading affect these radiative and jet efficiencies.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2015
- DOI:
- 10.1093/mnrasl/slv115
- arXiv:
- arXiv:1508.02433
- Bibcode:
- 2015MNRAS.454L...6M
- Keywords:
-
- MHD;
- radiation: dynamics;
- accretion;
- accretion discs;
- Astrophysics - High Energy Astrophysical Phenomena;
- General Relativity and Quantum Cosmology
- E-Print:
- 5 pages, 4 figures, MNRAS letters, in press, Movies: http://www.youtube.com/playlist?list=PLwa71jI0sY_AD9e8-7DXmJm4AHWFdARtT