Spacetime in motion: an evolving relativistic binary black hole metric for GIZMO

TL;DR

This paper introduces a relativistic binary black hole metric implementation in the GIZMO code, enabling efficient simulations of gas dynamics near merging black holes with relativistic effects, validated against numerical relativity and Newtonian models.

Contribution

The authors develop and validate a superposed Kerr-Schild dynamic metric in GIZMO for simulating relativistic black hole binaries with gas, improving accuracy and efficiency.

Findings

Good agreement with numerical relativity in relativistic Bondi flow

Moderate differences in accretion rates compared to Newtonian models

Relativistic effects influence shock formation and angular momentum transport

Abstract

The last evolutionary stages of massive black hole binaries prior to coalescence is dominated by the emission of gravitational waves, which will be probed by the future Laser Interferometer Space Antenna. If gas is present around the two black holes, however, the associated electromagnetic emission can provide additional information about the binary properties and location before the merger event. For this reason, a proper characterisation of the electromagnetic emission during these phases is of fundamental importance, and requires a detailed description of the gas dynamics close to the event horizon of the two black holes, only achievable via numerical simulations. Within this context, we present the implementation of the Superposed Kerr-Schild dynamic metric in the relativistic scheme in the meshless code GIZMO. Our code can now simulate black hole binaries approaching merger with high computational efficiency and accuracy, taking into account relativistic effects on the gas. To validate our implementation, we perform two tests. First, we explore the case of a relativistic Bondi flow around a binary, finding very good agreement with numerical relativity simulations. Then we explore the case of an inviscid relativistic circumbinary disc, comparing our results with a similar simulation run assuming Newtonian gravity. In this second case, we find moderate differences in the mass accretion rate and in the inflow dynamics, which suggest that the presence of a non-Keplerian potential and of apsidal precession in the orbiting gas trajectories may produce stronger shocks and boost angular momentum transport in the disc. Our work highlights the importance of accounting for relativistic corrections in accretion disc simulations around black hole binaries approaching merger, even at scales much larger than those currently probed by numerical relativity simulations.