The local stability of the magnetized advection-dominated discs with the radial viscous force

TL;DR

This paper investigates how magnetic fields and radial viscous forces influence the local stability of advection-dominated accretion discs, revealing their effects on growth rates of thermal and acoustic instabilities and implications for black hole QPOs.

Contribution

It provides a linear stability analysis showing the impact of magnetic fields and radial viscous forces on disc instabilities, highlighting their roles in thermal and acoustic mode growth rates.

Findings

Radial viscous force decreases the growth rate of outward-propagating acoustic modes.

Radial viscous force increases the thermal instability growth rate in slim discs.

Stronger magnetic fields suppress thermal mode instability in slim discs.

Abstract

We study local stability of the advection-dominated optically thick (slim) and optically thin discs with purely toroidal magnetic field and the radial viscous force using a linear perturbation analysis. Our dispersion relation indicates that the presence of magnetic fields and radial viscous force cannot give rise to any new mode of the instability. We find, however, that growth rate of the thermal mode in the slim discs and that of the acoustic modes in the slim and optically thin discs are dramatically affected by the radial viscous force. This force tends to strongly decrease the growth rate of the outward-propagating acoustic mode (O-mode) at the short-wavelength limit, but it causes a slim disc to become thermally more unstable. We find that growth rate of the thermal mode increases in the presence of radial viscous force. This enhancement is more significant when the viscosity parameter is large. We also show that growth rate of the O-mode reduces when radial viscous force is considered. The growth rates of the thermal and acoustic modes depend on the magnetic field. Although the instability of O-mode for a stronger magnetic field case has a higher growth rate, the thermal mode of the slim discs can be suppressed when the magnetic field is strong. The inertial-acoustic instability of a magnetized disc may explain the quasi-periodic oscillations (QPOs) from the black holes.