Accurate, long-term binary neutron stars simulations with IllinoisGRMHD and HARM3D

We perform binary neutron star (BNS) merger simulations in full dynamical general relativity with IllinoisGRMHD, on Cartesian AMR grid. After the remnant black hole (BH) has become nearly stationary, continuing to simulate the surrounding accretion disk on Cartesian grids over long timescales (~1s) would be suboptimal, as the plasma flows obliquely across coordinate lines and numerical errors quickly sap angular momentum from the disk. To address this, we hand off the GRMHD and spacetime data from IllinoisGRMHD to HARM3D, a GRMHD code that specializes in modeling BH accretion disks in static spacetimes over long timescales on spherical-like coordinate grids. We demonstrate a smooth and reliable transition of GRMHD fields and spacetime data, enabling us to efficiently and reliably evolve BNS dynamics for the needed longer timescales. We also discuss future plans, which involve incorporating advanced equations of state and neutrino physics into BNS simulations using this approach.