Gas squeezing during the merger of a supermassive black hole binary

TL;DR

This study uses 3D simulations to reveal a gas squeezing phenomenon during supermassive black hole mergers, leading to super-Eddington accretion rates and potential electromagnetic precursors to gravitational wave signals.

Contribution

It demonstrates the existence of a gas squeezing effect in 3D simulations, contrasting previous 2D results, and explores its implications for electromagnetic signals during black hole mergers.

Findings

Accretion rates exceed the Eddington limit during merger.

Gas squeezing causes increased luminosity as a precursor signal.

Negligible mass expulsion from the secondary's orbit in low aspect ratio discs.

Abstract

We study accretion rates during the gravitational wave-driven merger of a binary supermassive black hole embedded in an accretion disc, formed by gas driven to the centre of the galaxy. We use 3D simulations performed with PHANTOM, a Smoothed Particle Hydrodynamics code. Contrary to previous investigations, we show that there is evidence of a "squeezing phenomenon", caused by the compression of the inner disc gas when the secondary black hole spirals towards the primary. This causes an increase in the accretion rates that always exceed the Eddington rate. We have studied the main features of the phenomenon for a mass ratio $q = 10^{-3}$ between the black holes, including the effects of numerical resolution, the secondary accretion radius and the disc thickness. With our disc model with a low aspect ratio, we show that the mass expelled from the orbit of the secondary is negligible ($< 5\%$ of the initial disc mass), different to the findings of previous 2D simulations with thicker discs. The increase in the accretion rates in the last stages of the merger leads to an increase in luminosity, making it possible to detect an electromagnetic precursor of the gravitational wave signal emitted by the coalescence.