No snow-plough mechanism during the rapid hardening of supermassive black hole binaries

TL;DR

This study uses two-dimensional hydrodynamical simulations to challenge previous models, showing that the expected luminosity increase ('snow-plough' effect) during supermassive black hole binary mergers is unlikely due to effective mass leakage across the gap.

Contribution

It demonstrates that the snow-plough mechanism does not occur in realistic 2D models, revising prior predictions about luminosity signals before black hole mergers.

Findings

Mass leakage across the secondary's gap is effective.

Luminosity remains roughly constant prior to merger.

The snow-plough mechanism is unlikely to occur.

Abstract

We present two-dimensional hydrodynamical simulations of the tidal interaction between a supermassive black hole binary with moderate mass ratio, and the fossil gas disc where it is embedded. Our study extends previous one-dimensional height-integrated disc models, which predicted that the density of the gas disc between the primary and the secondary black holes should rise significantly during the ultimate stages of the binary's hardening driven by the gravitational radiation torque. This snow-plough mechanism, as we call it, would lead to an increase in the bolometric luminosity of the system prior to the binary merger, which could be detected in conjunction with the gravitational wave signal. We argue here that the snow-plough mechanism is unlikely to occur. In two-dimensions, when the binary's hardening timescale driven by gravitational radiation becomes shorter than the disc's viscous drift timescale, fluid elements in the inner disc get funneled to the outer disc through horseshoe trajectories with respect to the secondary. Mass leakage across the secondary's gap is thus found to be effective and, as a result, the predicted accretion disc luminosity will remain at roughly the same level prior to merger.