Efficiency of Super-Eddington Magnetically-Arrested Accretion
Abstract
The radiative efficiency of super-Eddington accreting black holes (BHs) is explored for magnetically-arrested disks (MADs), 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\% when time-averaged and total efficiency 100\% in moments. Magnetic compression by the magnetic flux near the rotating BH leads to a thin disk, 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\% on the horizon and BZ efficiency 5\% by r 400rg (gravitational radii) via absorption by the wind. Importantly, radiation escapes at r 400rg with efficiency η≈ 15\% (luminosity L 50L Edd), similar to η≈ 12\% for a Novikov-Thorne thin disk and beyond η 1\% seen in prior GRRMHD simulations or slim disk theory. Our simulations show how BH spin, magnetic field, and jet mass-loading affect the radiative and jet efficiencies of super-Eddington accretion.