No Evidence For Bardeen-Petterson Alignment In Conservative GRMHD Simulation Of Moderately Thin, Tilted Accretion Disk

 

Danilo Morales Teixeira (IAG/USP)

 

In this work we present our latest numerical simulations of accretion disks that are misaligned (tilted) with respect to the rotation axis of a Kerr black hole. In this work we use a new, fully conservative version of our general relativistic magneto-hydrodynamics (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 H/r of the disk become comparable. We also introduce the first results that use data extracted from numerical simulations as inputs to the analytic twisted disk model of Zhuravlev & Ivanov. The simulated disk shows no sign of Bardeen-Petterson alignment, consistent with our earlier work, but contrary to common expectations for thin disks. Importantly, we also find that the dimensionless stress resulting from the MRI-generated turbulence in our simulations is not isotropic, also contrary to common assumptions. Finally, we find that the predictions from the Zhuravlev & Ivanov model for the dynamics and stationary configuration of the disk agree quite well with the simulations. The implication is that the parameter space associated with Bardeen-Petterson alignment may be quite small, only including very thin disks H/r << alpha.