Eccentric Minidisks in Accreting Binaries

The Astrophysical Journal, vol. 962, pp. 76

Abstract

Abstract We show that gas disks around the components of an orbiting binary system (so-called minidisks) may be susceptible to a resonant instability that 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 Γ-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 (≳4% of the binary semimajor 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 ≲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 quasiperiodic 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.

Authors 4

1. John Ryan Westernacher-Schneider corresponding Aachen Leiden Observatory

   Leiden University · Leiden Observatory · Clemson University

   Affiliation as printed

   Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA

   Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands; john.westernacher.schneider@gmail.com

   Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands

2. Jonathan Zrake

   Clemson University

   Affiliation as printed

   Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA

3. Andrew I. MacFadyen

   New York University

   Affiliation as printed

   Center for Cosmology and Particle Physics, Physics Department, New York University, New York, NY 10003, USA

4. Zoltán Haiman

   Columbia University

   Affiliation as printed

   Department of Astronomy, Columbia University, New York, NY 10027, USA