Suppressed accretion onto massive black hole binaries surrounded by thin disks

TL;DR

This study shows that gas disks around massive black hole binaries can suppress accretion when the disk is hot enough, leading to dimmer systems that may re-brighten after black hole merger, affecting their observational signatures.

Contribution

It introduces new hydrodynamics simulations revealing conditions under which accretion onto black hole binaries is suppressed due to high Mach numbers and thermodynamic effects.

Findings

Accretion stops when Mach number exceeds ~40 in the disk.

High Mach number disks lead to mass pile-up in a surrounding ring.

Suppressed accretion results in dimmer electromagnetic signatures.

Abstract

We demonstrate that gas disks around binary systems might deliver gas to the binary components only when the circumbinary disk is relatively warm. We present new grid-based hydrodynamics simulations, performed with the binary on the grid and a locally isothermal equation of state, in which the binary is seen to functionally ``stop accreting'' if the orbital Mach number in the disk exceeds a threshold value of about 40. Above this threshold, the disk continues to extract angular momentum from the binary orbit, but it delivers very little mass to the black holes, and instead piles up mass in a ring surrounding the binary. This ring will eventually become viscously relaxed and deliver mass to the binary at the large-scale inflow rate. However we show that the timescale for such relaxation can far exceed the implied binary lifetime. We demonstrate that the ability of a binary-disk system to equilibrate is dependent on the efficiency at which accretion streams deposit mass onto the binary; which in turn is highly sensitive to the thermodynamic conditions of the inner disk. If disks around massive black hole binaries do operate in such non-accreting regimes, it suggests these systems may be dimmer than their single black hole counterparts, but could exhibit dramatic re-brightening after the black holes in-spiral and merge. This dimming begins in the UV/optical and could completely choke high-energy emission, such that these systems would likely be intrinsically X-ray weak with reddened continua, potentially resembling the spectra of `Little Red Dots'' recently identified in JWST observations.