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The last evolutionary stages of massive black hole binaries prior to coalescence are 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, 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; this is 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 a merger with high computational efficiency and accuracy, taking relativistic effects on the gas into account. To validate our implementation, we performed two tests. First, we explored the case of a relativistic Bondi flow around a binary, finding very good agreement with numerical relativity simulations. Then, we explored 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 apsidal precession in the orbiting gas trajectories 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 a merger, even at scales much larger than those currently probed by numerical relativity simulations.
Fedrigo et al. (Sun,) studied this question.