Global simulations of axisymmetric radiative black hole accretion disks in general relativity with a sub-grid magnetic dynamo

TL;DR

This paper introduces a sub-grid magnetic dynamo model in GRRMHD simulations, enabling long-term 2D axisymmetric studies of super-Eddington black hole accretion disks, revealing high efficiencies and jet collimation.

Contribution

The paper develops and implements a new sub-grid dynamo model in GRRMHD simulations, allowing for sustained long-term axisymmetric modeling of accretion disks with realistic magnetic turbulence.

Findings

Super-Eddington disks around non-spinning black holes have efficiency ~0.04.

Spinning black hole disks extract rotational energy, increasing efficiency.

Disks with sub-Eddington accretion rates tend to collapse due to thermal instability.

Abstract

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