Eccentric Minidisks in Accreting Binaries

TL;DR

This paper investigates the potential for eccentric minidisks in accreting binary systems due to a resonant instability, highlighting the conditions under which eccentricity develops and its possible observational implications.

Contribution

The study demonstrates that eccentric minidisks can form through a resonant instability in simulations, and explores how numerical and physical conditions influence their development.

Findings

Eccentric minidisks arise from a resonant instability in simulations.

Eccentricity is suppressed by isothermal equations of state and large gravitational softening.

Eccentric minidisks could impact electromagnetic signals and gravitational wave detection.

Abstract

We show that gas disks around the components of an orbiting binary system (so-called minidisks) may be susceptible to a resonant instability which causes the minidisks to become significantly eccentric. Eccentricity is injected by, and also induces, regular impacts between the minidisks at roughly the orbital period of the binary. Such eccentric minidisks are seen in vertically integrated, two-dimensional simulations of a circular, equal-mass binary accreting from a circumbinary gas disk with a $\Gamma$-law equation of state. Minidisk eccentricity is suppressed by the use of an isothermal equation of state. However, the instability still operates, and can be revealed in a minimal disk-binary simulation by removing the circumbinary disk, and feeding the minidisks from the component positions. Minidisk eccentricity is also suppressed when the gravitational softening length is large ($\gtrsim 4\%$ of the binary semi-major axis), suggesting that its absence could be an artifact of widely adopted numerical approximations; a follow-up study in three dimensions with well-resolved, geometrically thin minidisks (aspect ratios $\lesssim 0.02$) may be needed to assess whether eccentric minidisks can occur in real astrophysical environments. If they can, the electromagnetic signature may be important for discriminating between binary and single black hole scenarios for quasi-periodic oscillations in active galactic nuclei; in turn, this might aid in targeted searches with pulsar timing arrays for individual supermassive black hole binary sources of low-frequency gravitational waves.