Delayed jet launching in binary neutron star mergers with realistic initial magnetic fields

TL;DR

This study uses large eddy simulations to analyze magnetic field evolution in binary neutron star mergers, revealing a helicoidal structure and emphasizing the importance of initial magnetic field configuration for accurate modeling.

Contribution

It introduces a realistic initial magnetic field setup that better replicates turbulent merger conditions, improving simulation accuracy without high computational costs.

Findings

Helicoidal magnetic field structure governed by toroidal component.

Significant increase in poloidal magnetic field at late times.

Isotropic small-scale magnetic fields better match high-resolution results.

Abstract

We analyze a long-lived hyper-massive neutron star merger remnant (post-merger lifetime $>250$ ms) that has been obtained via large eddy simulations with a gradient subgrid-scale model. We find a clear helicoidal magnetic field structure that is governed by the toroidal component of the magnetic field. Although no jet emerges during the simulation time, we observe at late times a significant increase of the poloidal component of the magnetic field at all scales. We also compare with the results of several binary neutron star simulations with moderate resolution of $120$~m, that are evolved up to $50$~ms after the merger, which differ in terms of the initial topology and strength of the magnetic field. We find that the best choice is an isotropic small-scale magnetic field distribution that mimics the turbulent state that generically develops during the merger. This initial configuration reaches a closer agreement with our high-resolution simulation results than the purely dipolar large-scale fields that are commonly employed in these type of simulations. This provides a recipe to perform such simulations avoiding the computationally expensive grids required to faithfully capture the amplification of the magnetic field by Kelvin-Helmholtz instabilities.