Conservative GRMHD Simulations of Moderately Thin, Tilted Accretion Disks

TL;DR

This study uses a new conservative GRMHD code with an ad hoc cooling function to simulate the thinnest tilted accretion disks around Kerr black holes, revealing insights into disk alignment and shock formation.

Contribution

Introduces a fully conservative GRMHD simulation method with cooling to model very thin, tilted accretion disks, exploring their alignment behavior and turbulence characteristics.

Findings

Prograde disks show no Bardeen-Petterson alignment.

Retrograde disks exhibit modest alignment.

No standing shocks observed in simulations.

Abstract

This paper presents our latest numerical simulations of accretion disks that are misaligned with respect to the rotation axis of a Kerr black hole. In this work we use a new, fully conservative version of the Cosmos++ general relativistic magnetohydrodynamics (GRMHD) code, coupled with an ad hoc cooling function designed to control the thickness of the disk. Together these allow us to simulate the thinnest tilted accretion disks ever using a GRMHD code. In this way, we are able to probe the regime where the dimensionless stress and scale height of the disk become comparable. We present results for both prograde and retrograde cases. The simulated prograde tilted disk shows no sign of Bardeen-Petterson alignment even in the innermost parts of the disk. The simulated retrograde tilted disk, however, does show modest alignment. The implication of these results is that the parameter space associated with Bardeen-Petterson alignment for prograde disks may be rather small, only including very thin disks. Unlike our previous work, we find no evidence for standing shocks in our simulated tilted disks. We ascribe this to the combination of small black hole spin, small tilt angle, and small disk scale height in these simulations. We also add to the growing body of literature pointing out that the turbulence driven by the magnetorotational instability in global simulations of accretion disks is not isotropic. Finally, we provide a comparison between our moderately thin, untilted reference simulation and other numerical simulations of thin disks in the literature.