Warped Circumbinary Disks in Active Galactic Nuclei

TL;DR

This paper investigates the warping instability of circumbinary disks around binary supermassive black holes, highlighting the role of radiative torques and the conditions under which the disks become warped, with implications for observed warped maser disks.

Contribution

It introduces a model for radiation-driven warping of circumbinary disks, considering the effects of tidal and radiative torques, and estimates the warping radius and its implications for black hole binaries.

Findings

Circumbinary disks are unstable to radiation-driven warping beyond a certain radius.

Tidal torques tend to align the disk with the binary orbital plane.

Warping can occur at radii of about 0.1 pc, suggesting binary black holes in observed warped disks.

Abstract

We study a warping instability of a geometrically thin, non-self-gravitating disk surrounding binary supermassive black holes on a circular orbit. Such a circumbinary disk is subject to not only tidal torques due to the binary gravitational potential but also radiative torques due to radiation emitted from an accretion disk around each black hole. We find that a circumbinary disk initially aligned with the binary orbital plane is unstable to radiation-driven warping beyond the marginally stable warping radius, which is sensitive to both the ratio of vertical to horizontal shear viscosities and the mass-to-energy conversion efficiency. As expected, the tidal torques give no contribution to the growth of warping modes but tend to align the circumbinary disk with the orbital plane. Since the tidal torques can suppress the warping modes in the inner part of circumbinary disk, the circumbinary disk starts to be warped at radii larger than the marginally stable warping radius. If the warping radius is of the order of 0.1 pc, a resultant semi-major axis is estimated to be of the order of 10^-2 pc to 10^-4 pc for 10^7 Msun black hole. We also discuss the possibility that the central objects of observed warped maser disks in active galactic nuclei are binary supermassive black holes with a triple disk: two accretion disks around the individual black holes and one circumbinary disk surrounding them.