Efficiency of thin magnetically arrested discs around black holes

The radiative and jet efficiencies of thin magnetized accretion discs 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 disc theory. To seek the maximum deviations, we perform general relativistic magnetohydrodynamic simulations of radiatively efficient thin (half-height H to radius R of H/R approximate to 0.10) discs around moderately rotating BHs with a/M = 0.5. First, our simulations, each evolved for more than 70 000 r(g)/c (gravitational radius r(g) and speed of light c), show that large-scale magnetic field readily accretes inward even through our thin disc and builds-up to the magnetically arrested disc (MAD) state. Secondly, our simulations of thin MADs show the disc achieves a radiative efficiency of eta(r) approximate to 15 per cent (after estimating photon capture), which is about twice the NT value of eta(r) similar to 8 per cent for a/M = 0.5 and gives the same luminosity as an NT disc with a/M approximate to 0.9. Compared to prior simulations with less than or similar to 10 per cent deviations, our result of an approximate to 80 per cent deviation sets a new benchmark. Building on prior work, we are now able to complete an important scaling law which suggests 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.