Gas-driven inspiral of binaries in thin accretion disks

TL;DR

This study systematically explores how the orbital evolution of supermassive black hole binaries in accretion disks depends on disk thickness, revealing a transition from outspiral to inspiral at realistic thin disk conditions and implications for gravitational wave predictions.

Contribution

First systematic investigation of binary orbital evolution dependence on disk scale height, highlighting a critical aspect ratio where behavior shifts from outspiral to inspiral.

Findings

Binary switches from outspiral to inspiral at aspect ratio ~0.04

Net torque is influenced by a competition between positive and negative contributions

Accretion rate decreases modestly with increasing Mach number

Abstract

Numerical studies of gas accretion onto supermassive black hole binaries (SMBHBs) have generally been limited to conditions where the circumbinary disk (CBD) is 10-100 times thicker than expected for disks in active galactic nuclei (AGN). This discrepancy arises from technical limitations, and also from publication bias toward replicating fiducial numerical models. Here we present the first systematic study of how the binary's orbital evolution varies with disk scale height. We report three key results: (1) Binary orbital evolution switches from outspiralling for warm disks (aspect ratio ~0.1), to inspiralling for more realistic cooler, thinner disks at a critical aspect ratio ~0.04, corresponding to orbital Mach number ~25. (2) The net torque on the binary arises from a competition between positive torque from gas orbiting close to the black holes, and negative torque from the inner edge of the CBD, which is denser for thinner disks. This leads to increasingly negative net torques on the binary for increasingly thin disks. (3) The accretion rate is modestly suppressed with increasing Mach number. We discuss how our results may influence modeling of the nano-Hz gravitational wave background, as well as estimates of the LISA merger event rate.