GRMHD simulations of accretion flows onto massive binary black hole mergers embedded in a thin slab of gas

TL;DR

This study uses GRMHD simulations to analyze accretion flows onto merging binary black holes in a thin gas slab, revealing spin-dependent accretion modulations and no post-merger luminosity peak, impacting electromagnetic detection strategies.

Contribution

It provides the first detailed GRMHD simulation of binary black hole mergers in a thin gas slab, highlighting the influence of spin and initial gas geometry on accretion and luminosity signatures.

Findings

Spin induces accretion rate modulations proportional to orbital frequency.

No significant increase in accretion rate after merger observed.

Gas geometry influences electromagnetic signatures and potential observability.

Abstract

We present general relativistic magnetohydrodynamic simulations of merging equal-mass, spinning black holes embedded in an equatorial thin slab of magnetized gas. We evolve black holes either non-spinning, with spins aligned to the orbital angular momentum, and with misaligned spins. The rest-mass density of the gas slab follows a Gaussian profile symmetric relative to the equatorial plane and it is initially either stationary or with Keplerian rotational support. As part of our diagnostics, we follow the accretion of matter onto the black hole horizons and the Poynting luminosity. Throughout the inspiral phase, the configurations with non-zero spins display modulations in the mass accretion rate that are proportional to the orbital frequency and its multiples. Our frequency analysis suggests that these modulations are influenced by the initial geometry and angular momentum of the gas distribution. In contrast to binary models evolved in the gas cloud scenario, we do not observe a significant increase in the mass accretion rate after the merger in any of our simulations. This observation brings attention to a potential link between the electromagnetic signatures of massive binary black hole mergers and the geometrical distribution of the surrounding gas. It also suggests the possibility of not detecting a peak luminosity at the time of merger in future electromagnetic observations.