Consequences of Low Resolution and High Initial Magnetic Fields in Binary Neutron Star Merger Simulations

          @misc{ pirsa_PIRSA:25030145,
            doi = {10.48660/25030145},
            url = {https://pirsa.org/25030145},
            author = {},
            keywords = {},
            language = {en},
            title = {Consequences of Low Resolution and High Initial Magnetic Fields in Binary Neutron Star Merger Simulations},
            publisher = {Perimeter Institute},
            year = {2025},
            month = {mar},
            note = {PIRSA:25030145 see, \url{https://pirsa.org}}
          }
          

Abstract

Simulations of magnetized binary neutron star mergers often seed the interiors of the initial stars with unrealistically strong magnetic fields to overcome the suppression of small-scale turbulence by finite grid resolution and observe postmerger magnetic collimation and potential jet breakout. We present a curious numerical instability arising from low resolution (227 meters) and high initial dipolar fields ($E_{tot} = 5 \times 10^{49}$ ergs) observed when conducting BNS mergers in a full 3D domain. Initial poloidal structures of sufficient magnitude can linger within the merger remnant, even through the turbulent merger process. Differential rotation then winds these structures into two counterrotating torii separated by the x-y plane. Numerical diffusion inherent to the low resolution grid then causes counterrotating field lines to interact near the x-y plane, leading to spurious magnetic energy dissipation that feeds back into fluid motion. We discuss the consequences of this feedback, including a large circular drift of the merger remnant, and how increased resolution or grid symmetry can alleviate this issue.