On the dynamics of low-viscosity warped discs around black holes

TL;DR

This paper investigates the behavior of low-viscosity warped accretion discs around black holes, demonstrating through simulations and theory that inner disc tilt can be significantly amplified, with implications for disc tearing and observational signatures.

Contribution

It provides the first validation of linearised fluid equations with 3D hydrodynamical simulations for low-viscosity warped discs, revealing tilt amplification and disc tearing phenomena.

Findings

Inner disc tilt can be several times larger than initial tilt

Disc tearing occurs at low viscosity and high warp amplitude

High-resolution simulations are essential for accurate warp modeling

Abstract

Accretion discs around black holes can become warped by Lense-Thirring precession if the disc is tilted with respect to the black hole spin vector. When the disc viscosity is sufficiently large that warp propagation is diffusive, the inner disc can align with the black hole spin. However, if the viscosity is small, such that the warp propagates as a wave, then steady-state solutions to the linearised fluid equations exhibit an oscillatory radial profile of the disc tilt angle close to the black hole. Here we show, for the first time, that these solutions are in good agreement with three-dimensional hydrodynamical simulations, in which the viscosity is isotropic and measured to be small compared to the disc angular semi-thickness, and in the case that the disc tilt -- and thus the warp amplitude -- remains small. We show using both the linearised fluid equations and hydrodynamical simulations that the inner disc tilt can be more than several times larger than the original disc tilt, and we provide physical reasoning for this effect. We explore the transition in disc behaviour as the misalignment angle is increased, finding increased dissipation associated with regions of strong warping. For large enough misalignments the disc becomes unstable to disc tearing and breaks into discrete planes. For the simulations we present here, we show that the total (physical and numerical) viscosity at the time the disc breaks is small enough that the disc tearing occurs in the wave-like regime, substantiating that disc tearing is possible in this region of parameter space. Our simulations demonstrate that high spatial resolution, and thus low numerical viscosity, is required to accurately model the warp dynamics in this regime. Finally, we discuss the observational implications of our results.