HARM3D+NUC: A new method for simulating the post-merger phase of binary neutron star mergers with GRMHD, tabulated EOS and neutrino leakage

TL;DR

This paper introduces HARM3D+NUC, a GRMHD simulation code that incorporates a finite-temperature EOS and neutrino leakage, enabling realistic modeling of post-merger neutron star disk outflows and nucleosynthesis.

Contribution

It presents the implementation and validation of a new GRMHD code with advanced physics for simulating neutron star merger remnants.

Findings

Code successfully reproduces nucleon recombination to neutron-rich composition.

Code accurately models thermal wind excitation in remnant disks.

Validates the inclusion of finite-temperature EOS and neutrino leakage in simulations.

Abstract

The first binary neutron star merger has already been detected in gravitational waves. The signal was accompanied by an electromagnetic counterpart including a kilonova component powered by the decay of radioactive nuclei, as well as a short $\gamma$-ray burst. In order to understand the radioactively-powered signal, it is necessary to simulate the outflows and their nucleosynthesis from the post-merger disk. Simulating the disk and predicting the composition of the outflows requires general relativistic magnetohydrodynamical (GRMHD) simulations that include a realistic, finite-temperature equation of state (EOS) and self-consistently calculating the impact of neutrinos. In this work, we detail the implementation of a finite-temperature EOS and the treatment of neutrinos in the GRMHD code HARM3D+NUC, based on HARM3D. We include formal tests of both the finite-temperature EOS and the neutrino leakage scheme. We further test the code by showing that, given conditions similar to those of published remnant disks following neutron star mergers, it reproduces both recombination of free nucleons to a neutron-rich composition and excitation of a thermal wind.