Global simulations of axisymmetric radiative black hole accretion discs in general relativity with a mean-field magnetic dynamo

Abstract

We present a sub-grid model that emulates the magnetic dynamo operating in\nmagnetized accretion disks. We have implemented this model in the general\nrelativisic radiation magnetohydrodynamic (GRRMHD) code \\koral, using results\nfrom local shearing sheet simulations of the magnetorotational instability to\nfix the parameters of the dynamo. With the inclusion of this dynamo, we are\nable to run 2D axisymmetric GRRMHD simulations of accretion disks for\narbitrarily long times. The simulated disks exhibit sustained turbulence, with\nthe poloidal and toroidal magnetic field components driven towards a state\nsimilar to that seen in 3D studies. Using this dynamo code, we present a set of\nlong-duration global simulations of super-Eddington, optically-thick disks\naround non-spinning and spinning black holes. Super-Eddington disks around\nnon-rotating black holes exhibit a surprisingly large efficiency,\n$\\eta\\approx0.04$, independent of the accretion rate, where we measure\nefficiency in terms of the total energy output, both radiation and mechanical,\nflowing out to infinity. Super-Eddington disks around spinning black holes are\neven more efficient, and appear to extract black hole rotational energy through\na process similar to the Blandford-Znajek mechanism. All the simulated models\nare characterized by highly super-Eddington radiative fluxes collimated along\nthe rotation axis. We also present a set of simulations that were designed to\nhave Eddington or slightly sub-Eddington accretion rates ($\\dot{M} \\lesssim\n2\\dot M_{\\rm Edd}$). None of these models reached a steady state. Instead, the\ndisks collapsed as a result of runaway cooling, presumably because of a thermal\ninstability.\n