Efficiency of Thin Magnetically-Arrested Disks Around Black Holes

TL;DR

This study uses GRMHD simulations to show that thin, magnetically-arrested accretion disks around black holes can achieve radiative efficiencies significantly higher than traditional models, impacting our understanding of black hole accretion and jet formation.

Contribution

First simulation of thin MADs around black holes demonstrating efficiencies twice that of Novikov-Thorne predictions, establishing a new benchmark for deviations.

Findings

Thin MAD disks reach ~15% radiative efficiency, about twice NT predictions.

Simulations show magnetic fields readily build up to MAD state in thin disks.

Results support the presence of persistent MAD states influencing jet quenching in X-ray binaries.

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.