No Evidence for Bardeen-Petterson Alignment in GRMHD Simulations and Semi-Analytic Models of Moderately Thin, Prograde, Tilted Accretion Disks

TL;DR

This study investigates whether the Bardeen-Petterson alignment occurs in tilted accretion disks around black holes, finding no evidence for it in prograde cases with moderate viscosity through combined numerical simulations and analytic models.

Contribution

It provides the first direct comparison of GRMHD simulations with an analytic twisted disk model, showing the absence of Bardeen-Petterson alignment in prograde disks with moderate viscosity.

Findings

No Bardeen-Petterson alignment observed in prograde disks

Partial alignment occurs when disk rotation opposes black hole spin

Analytic and numerical models show consistent qualitative behavior

Abstract

In this paper we introduce the first results that use data extracted directly from numerical simulations as inputs to the analytic twisted disk model of Zhuravlev & Ivanov. In both numerical and analytic approaches, fully relativistic models of tilted and twisted disks having a moderate effective viscosity around a slowly rotating Kerr black hole are considered. Qualitatively, the analytic model demonstrates the same dynamics as the simulations, although with some quantitative offset. Namely, the GRMHD simulations generally give smaller variations of tilt and twist across the disk. When the black hole and the disk rotate in the same sense, the simulated tilted disk and analytic model show no sign of Bardeen-Petterson alignment, even in the innermost parts of the disk, where the characteristic time for relaxation to a quasi-stationary configuration is of the same order as the computation time. In the opposite case, when the direction of the disk's rotation is opposite to that of black hole, a partial alignment is observed, in agreement with previous theoretical estimates. Thus, both fully numerical and analytic schemes demonstrate that the Bardeen-Petterson effect may not be possible for the case of prograde rotation provided that disk's effective viscosity is sufficiently small. This may have implications in modeling of different astrophysical phenomena, such as disk spectra and jet orientation.