Upgrading SPHINCS_BSSN: Black Hole–Neutron Star Initial Data for Binary Merger Simulations

For current-generation gravitational wave detectors - LIGO, Virgo, and KAGRA (LVK) - compact binary mergers (CBMs) are the primary sources of detectable signals. Robust and accurate simulations of these events are essential for deepening our understanding of CBMs and the signals that they produce. The SPHINCS_BSSN code distinguishes itself from other numerical relativity codes by evolving the matter of compact objects - like neutron stars - using Lagrangian particles via the smoothed-particle hydrodynamics (SPH) method. In numerical relativity, initial data (ID) for the spacetime geometry and matter distribution is obtained by solving a system of constraint equations. To do this, we use an external spectral solver from the Frankfurt University/Kadath (FUKA) collection. Our SPHINCS_ID code processes the solution from FUKA to generate an initial distribution of SPH particles and exports both the matter and spacetime ID to SPHINCS_BSSN for evolution. Originally, SPHINCS_ID supported only binary neutron star (BNS) systems. In this project, we extend its functionality to prepare ID for black hole–neutron star (BHNS) binaries to enable broader comparison with CBM events observed by LVK.