Warping and tearing of misaligned circumbinary disks around eccentric SMBH binaries

TL;DR

This paper investigates how misaligned circumbinary disks around eccentric SMBH binaries warp and tear due to tidal torques, deriving critical radii and conditions for different disk behaviors, with implications for observations.

Contribution

It derives the tidal-warp and tearing radii for misaligned disks around eccentric SMBH binaries and identifies conditions under which warping or tearing occurs, linking disk features to binary parameters.

Findings

Warping occurs outside the tearing radius if alpha > sqrt{H/3r}.

Only tearing occurs if alpha < sqrt{H/3r}.

Semi-major axis can be estimated from observed warp or tear radii.

Abstract

We study the warping and tearing of a geometrically thin, non-self-gravitating disk surrounding binary supermassive black holes on an eccentric orbit. The circumbinary disk is significantly misaligned with the binary orbital plane, and is subject to the time-dependent tidal torques. In principle, such a disk is warped and precesses, and is torn into mutually misaligned rings in the region, where the tidal precession torques are stronger than the local viscous torques. We derive the tidal-warp and tearing radii of the misaligned circumbinary disks around eccentric SMBH binaries. We find that in disks with the viscosity parameter, alpha, larger than a critical value depending on the disk aspect ratio, the disk warping appears outside the tearing radius. This condition is expressed as alpha > sqrt{H/3r} for H/r ~<0.1, where H is the disk scale height. If alpha < sqrt{H/3r}, only the disk tearing occurs because the tidal warp radius is inside the tearing radius, where most of disk material is likely to rapidly accrete onto SMBHs. In warped and torn disks, both the tidal-warp and the tearing radii most strongly depend on the binary semi-major axis, although they also mildly depend on the other orbital and disk parameters. This strong dependence enables us to estimate the semi-major axis, once the tidal warp or tearing radius is determined observationally: For the tidal warp radius of 0.1 pc, the semi-major axis is estimated to be ~10^{-2} pc for 10^7 Msun black hole with typical orbital and disk parameters. We also briefly discuss the possibility that central objects of observed warped maser disks in active galactic nuclei are supermassive black hole binaries.