A Reproducible Black Hole-Neutron Star Merger Gallery Example for the Einstein Toolkit

Abstract

Black hole-neutron star mergers, together with binary neutron star mergers, are key laboratories for neutron star physics. They enable us to probe merger dynamics imprinted in gravitational waves and potential electromagnetic counterparts. These systems link microphysics and macrophysics by placing constraints on the dense matter equations of state, potentially revealing the imprint of hadron-quark phase transitions, clarifying the role of neutrino irradiation in shaping the ejecta, its r-process nucleosynthesis, and kilonova emission, as well as assessing how magnetically driven instabilities affect mass ejection and possible electromagnetic signatures. Despite their importance, black hole-neutron star mergers remain relatively less studied and therefore not yet well understood, largely due to the lack of publicly available numerical relativity setups suitable for such investigations. In this work, we present a fully reproducible black hole-neutron star merger simulation performed exclusively using Einstein Toolkit thorns, targeting the detected event \texttt{GW230529}. The simulations are carried out at three resolutions with finest grid spacings of $162$, $222$ and $310$ meters to assess numerical robustness. The entire setup, from initial data to a parameter file with some of the analysis scripts, is publicly released as a new Einstein Toolkit gallery example and will be distributed as part of the Hypatia release, establishing a reference black hole-neutron star merger configuration within the Einstein Toolkit.

Figures (7)

• Figure 1: Snapshots of the rest-mass density on the equatorial plane from the medium resolution simulation before, during and after the merger. The panels show the system at the start of the simulation (left), during the merger when the NS is tidally disrupted (middle), and the remnant surrounded by a disc (right). Times shown are given relative to the merger time.
• Figure 2: Volume-weighted root mean square norm of the Hamiltonian constraint (top panel), and GW strain extracted at $\sim738\, \mathrm{km}$ (bottom panel). The constraint violation decreases with increasing resolution, while the strain shows good agreement between HR and MR, with phase differences present at LR. The panels are aligned at the merger time, and HR, MR, and LR represent high-, medium-, and low-resolution simulations, respectively.
• Figure 3: Same figure as Fig. \ref{['fig:ham_strain']}, but now comparing BSSN and CCZ4 at medium resolution. CCZ4 exhibits lower constraint violations and only a very small phase deviation during the inspiral, whereas the phase difference increases in the post-merger phase.
• Figure 4: Convergence plots for the fourth- and fifth-order behaviour of the GW strain. Here, $h_l$, $h_m$, and $h_h$ denote the GW strain at low, medium, and high resolutions, respectively, while $Q_{4}$ and $Q_{5}$ represent the fourth- and fifth-order convergence factors.
• Figure 5: GW spectra for the $(l, m) = (2,2)$ mode at a distance of $200\, \mathrm{Mpc}$, compared to the sensitivity curves of the Advanced LIGO (04 and 05) and the Einstein Telescope (ET). HR, MR, and LR represent high-, medium-, and low-resolution simulations, respectively.