General relativistic radiation magnetohydrodynamic simulations of thin magnetically arrested discs

The classical, relativistic thin-disc theory of Novikov and Thorne (NT) predicts a maximum accretion efficiency of 40 per cent for an optically thick, radiatively efficient accretion disc around a maximally spinning black hole (BH). However, when a strong magnetic field is introduced to numerical simulations of thin discs, large deviations in efficiencies are observed, in part due to mass and energy carried by jets and winds launched by the disc or BH spin. The total efficiency of accretion can be significantly enhanced beyond that predicted by NT but it has remained unclear how the radiative component is affected. In order to study the effect of a dynamically relevant large-scale magnetic field on radiatively efficient accretion, we have performed numerical 3D general relativistic - radiative - magnetohydrodynamic (GRRMHD) simulations of a disc with scale height to radius ratio of H/R ~ 0.1 around a moderately spinning BH (a = 0.5) using the code HARMRAD. Our simulations are fully global and allow us to measure the jet, wind, and radiative properties of a magnetically arrested disc (MAD) that is kept thin via self-consistent transport of energy by radiation using the M1 closure scheme. Our fiducial disc is MAD out to a radius of ~16Rg and the majority of the total ~13 per cent efficiency of the accretion flow is carried by a magnetically driven wind. We find that the radiative efficiency is slightly suppressed compared to NT, contrary to prior MAD GRMHD simulations with an ad hoc cooling function, but it is unclear how much of the radiation and thermal energy trapped in the outflows could ultimately escape. © 2018 The Author(s).