Efficiency of Thin Magnetically-Arrested Disks Around Black Holes

Abstract: The radiative and jet efficiencies of thin magnetized accretion disks around black holes (BHs) are affected by BH spin and the presence of a magnetic field that, when strong, could lead to large deviations from Novikov-Thorne (NT) thin disk theory. To seek the maximum deviations, we perform general relativistic magnetohydrodynamic (GRMHD) simulations of radiatively efficient thin (half-height $H$ to radius $R$ of $H/R\approx 0.10$) disks around moderately rotating BHs with $a/M=0.5$. First, our simulations, each evolved for more than $70,000r_g/c$ (gravitational radius $r_g$ and speed of light $c$), show that large-scale magnetic field readily accretes inward even through our thin disk and builds-up to the magnetically-arrested disk (MAD) state. Second, our simulations of thin MADs show the disk achieves a radiative efficiency of $\eta_{\rm r}\approx 15\%$ (after estimating photon capture), which is about twice the NT value of $\eta_{\rm r}\sim 8\%$ for $a/M=0.5$ and gives the same luminosity as a NT disk with $a/M\approx 0.9$. Compared to prior simulations with $\lesssim 10\%$ deviations, our result of an $\approx 80\%$ deviation sets a new benchmark. Building on prior work, we are now able to complete an important scaling law which suggest that observed jet quenching in the high-soft state in BH X-ray binaries is consistent with an ever-present MAD state with a weak yet sustained jet.