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 $\sim 50$ times Eddington shows a total efficiency $\sim 50\%$ when time-averaged and total efficiency $\gtrsim 100\%$ 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 $\sim 40\%$ on the horizon and BZ efficiency $\sim 5\%$ by $r\sim 400r_g$ (gravitational radii) via absorption by the wind. Importantly, radiation escapes at $r\sim 400r_g$ with efficiency $\eta\approx 15\%$ (luminosity $L\sim 50L_{\rm Edd}$), similar to $\eta\approx 12\%$ for a Novikov-Thorne thin disk and beyond $\eta\lesssim 1\%$ 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.Comment: 5 pages, 4 figures, MNRAS letters, in press, Movies: http://www.youtube.com/playlist?list=PLwa71jI0sY_AD9e8-7DXmJm4AHWFdARt