Large Eddy Simulations of Magnetized Mergers of Black Holes and Neutron Stars

The LIGO-Virgo-Kagra collaboration has observed gravitational waves consistent with the mergers of a black hole and a neutron star, namely GW200105 and GW200115, providing evidence for such cataclysmic events. Although no electromagnetic counterpart was reported for either of these two events, under certain conditions black hole--neutron star mergers are expected to form a significant accretion disk and to produce both a short gamma ray burst and a kilonova, much as observed in the binary neutron star merger GW170817. Here, we extend our publicly available code $\texttt{MHDuet}$ to study numerically the merger of a magnetized neutron star with a black hole. $\texttt{MHDuet}$ employs Large Eddy Simulation (LES) techniques to help capture the magnetic field amplification resulting from turbulence and other sub-grid scale dynamics in the post-merger stage. In particular, we simulate a merger with parameters favorable to producing an accretion disk, focusing on the formation and dynamics of the turbulent disk and the resulting magnetic field amplification. Following the tidal disruption and during the formation of the accretion disk, the magnetic field undergoes significant amplification driven by the Kelvin-Helmholtz instability, reaching strengths of more than $10^{14}\,\rm{G}$ from a realistic initial strength of $10^{11}\,\rm{G}$ in short timescales of approximately $20\,\rm{ms}$. Despite employing LES techniques with a finest resolution of $120\,\text{m}$ that is among the highest in black hole-neutron star mergers, it is still insufficient to demonstrate convergence of the magnetic field growth. Although the effects of the LES are here rather modest, we expect them to be more significant at higher resolution, as observed in binary neutron star merger simulations.