On the dynamics of magnetoviscous warped discs around compact objects

TL;DR

This paper reviews the dynamics of warped accretion discs around compact objects, highlighting how relativistic and magnetic effects influence warp modes, viscosity, and potential disc tearing in astrophysical systems.

Contribution

It introduces a semi-analytic framework incorporating relativistic and magnetic effects to analyze warp modes and viscosity in tilted accretion discs, revealing new warp behaviors.

Findings

Magnetic fields can create new warp modes and avoided crossings.

Intense magnetic fields can reduce perpendicular viscosity at sub-Eddington rates.

Critical magnetic strengths for disc tearing may be lower in certain systems.

Abstract

Accretion discs that are tilted with respect to their compact hosts can warp out-of-plane through general relativistic frame-dragging. Warp influences disc dynamics in ways that have been studied extensively, especially as regards instabilities that might lead to rapid angular-momentum cancellation between neighbouring rings of fluid and mass infall. We provide a review of warped-disc phenomena here, revisiting key hydrodynamical assumptions that impact calculations of the shear viscosity controlling instability thresholds. Relativistic effects at the level of gas-parcel orbits are included, as are external Lorentz forces applied by the compact primary's magnetic field. Semi-analytic analysis reveals that intense magnetic fields can bring about new branches of warp modes and avoided crossings that significantly reduce the perpendicular viscosity at sub-Eddington accretion rates. Critical strengths required for misaligned torques to tear a thin disc may thus relax for systems like neutron star X-ray binaries or radio-loud active galactic nuclei.