Warp diffusion in accretion discs: a numerical investigation

Abstract

In this paper we explore numerically the evolution of a warped accretion disc. Here, we focus here on the regime where the warp evolves diffusively. By comparing the numerical results to a simple diffusion model, we are able to determine the diffusion coefficient of the warp, $α_2$, as a function of the relevant disc parameters. We find that while in general the disc behaviour is well reproduced by the diffusion model and for relatively large viscosities the warp diffusion is well described by the linear theory (in particular confirming that the warp diffusion coefficient is inversely proportional to viscosity), significant non-linear effects are present as the viscosity becomes smaller, but still dominates over wave-propagation effects. In particular, we find that the inverse dependence of the diffusion coefficient on viscosity breaks down at low viscosities, so that $α_2$ never becomes larger than a saturation value $α_{\rm max}$ of order unity. This can have major consequences in the evolution of systems where a warped disc is present. In particular, it affects the location of the warp radius in the Bardeen-Petterson effect and therefore the spin up (or spin down) of supermassive black holes in the nuclei of galaxies. Additionally, we also find that while the rate of warp diffusion does not depend significantly on the detailed viscosity formulation, the rate of internal precession generated by the warp is strongly affected by it. Such effects should be considered with care when modeling the evolution of warped discs. This emphasises the need to test the above results using different numerical schemes, and with higher resolution, in order to investigate the degree to which numerical simulations are able to provide accurate modeling of the complex fluid dynamics of warped discs. (Abridged)