Accretion disc viscosity: a limit on the anisotropy

TL;DR

This paper constrains the effective viscosities in warped accretion discs using observations of Her X-1, linking disc precession to turbulence driven by magneto-rotational instability and comparing findings with theoretical and simulation results.

Contribution

It provides observational constraints on disc viscosities in warped accretion discs, informing the understanding of turbulence origins and challenging some recent simulation outcomes.

Findings

Constraints on effective viscosities in warped discs

Agreement with analytical and numerical MHD turbulence studies

Discrepancy with some recent global simulations

Abstract

Observations of warped discs can give insight into the nature of angular momentum transport in accretion discs. Only a few objects are known to show strong periodicity on long timescales, but when such periodicity is present it is often attributed to precession of the accretion disc. The X-ray binary Hercules X-1/HZ Herculis (Her X-1) is one of the best examples of such periodicity and has been linked to disc precession since it was first observed. By using the current best-fitting models to Her X-1, which invoke precession driven by radiation warping, I place a constraint on the effective viscosities that act in a warped disc. These effective viscosities almost certainly arise due to turbulence induced by the magneto-rotational instability. The constraints derived here are in agreement with analytical and numerical investigations into the nature of magneto-hydrodynamic disc turbulence, but at odds with some recent global simulations.