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

Giacomo Fedrigo, Alessandro Lupi, Alessia Franchini, Matteo Bonetti

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.

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

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.
Paper Structure (11 sections, 7 equations, 7 figures)

This paper contains 11 sections, 7 equations, 7 figures.

Table of Contents

  1. Introduction
  2. Black hole binary metric
  3. SKS formulation
  4. SKS implementation in gizmo
  5. Metric validation tests
  6. Bondi accretion
  7. Circumbinary discs evolution
  8. Post-Newtonian validation test
  9. Angular momentum transport and effective viscosity
  10. Minidiscs morphology and dynamics
  11. Conclusions

Figures (7)

  • Figure 1: Mass accretion rate as a function of time in the Bondi-like test. The solution from Cattorini2021 is marked with an orange line.
  • Figure 2: Density map of the central part of the circumbinary disk for the general relativistic run (left) and the Newtonian run (right), at $t=200T_0$. Density is volume-averaged along the z direction. The white circles mark the BHs' event horizons, scaled up by 10 times for visualisation purposes.
  • Figure 3: Surface density $\Sigma$ radial profile for the GR (blue), for the Newtonian (red), and for the Paczynski-Wiita (lime) runs at $t=200T_0$. The initial $\Sigma$ profile is marked with a black dashed line.
  • Figure 4: Mass accretion rate as a function of time for the GR (blue), the Newtonian (red), and the Paczynski-Wiita (lime) binaries. The transient phase, caused by the initial data relaxation, is marked with a grey shaded region.
  • Figure 5: Density-weighted gas temperature distribution at $t=200T_0$ for the GR (top panel) and the Newtonian (bottom panel) run. The inner cavity ($r\lesssim 225M$) is masked with a white patch. The temperature is normalized for a binary of $M_{b}=10^6 M_\odot$.
  • ...and 2 more figures