Nonlinear evolution of the magnetorotational instability in eccentric disks

TL;DR

This paper presents the first nonlinear simulations of the magnetorotational instability in eccentric disks, revealing similarities to circular disks but also unique behaviors like negative Maxwell stress in certain sectors, impacting accretion and energy efficiency.

Contribution

It provides novel insights into MRI behavior in eccentric disks through the first nonlinear simulation study, expanding understanding beyond circular disk models.

Findings

MRI in eccentric disks shows similar saturation levels to circular disks.

Maxwell stress can be negative in some disk sectors.

Eccentric disks may have lower radiative efficiency, affecting energy output.

Abstract

The magnetorotational instability (MRI) has been extensively studied in circular magnetized disks, and its ability to drive accretion has been demonstrated in a multitude of scenarios. There are reasons to expect eccentric magnetized disks to also exist, but the behavior of the MRI in these disks remains largely uncharted territory. Here we present the first simulations that follow the nonlinear development of the MRI in eccentric disks. We find that the MRI in eccentric disks resembles circular disks in two ways, in the overall level of saturation and in the dependence of the detailed saturated state on magnetic topology. However, in contrast with circular disks, the Maxwell stress in eccentric disks can be negative in some disk sectors, even though the integrated stress is always positive. The angular momentum flux raises the eccentricity of the inner parts of the disk and diminishes the same of the outer parts. Because material accreting onto a black hole from an eccentric orbit possesses more energy than material tracing the innermost stable circular orbit, the radiative efficiency of eccentric disks may be significantly lower than circular disks. This may resolve the "inverse energy problem" seen in many tidal disruption events.