Magnetic Field Configurations in Binary Neutron Star Mergers II: Inspiral, Merger and Ejecta

TL;DR

This study uses simulations to analyze how initial magnetic field configurations affect gravitational wave signals, magnetic field amplification, and ejecta properties in binary neutron star mergers, revealing complex dependencies and symmetry-breaking phenomena.

Contribution

It provides new insights into the impact of initial magnetic topologies on merger dynamics, gravitational wave phase evolution, and ejecta magnetization, highlighting the role of symmetry breaking.

Findings

Magnetic field amplification varies with initial topology and can be enhanced or suppressed.

Gravitational wave phase evolution is sensitive to initial magnetic configurations and numerical schemes.

Ejecta magnetic fields are highly magnetized and randomly oriented, with symmetry breaking observed.

Abstract

We perform a series of simulations of magnetised Binary Neutron Star mergers, with varying magnetic field topologies in the initial data, as well as varying Equations of State, and mass ratios. In this paper, a companion paper to arXiv:2506.18995, we analyse the impact of the initial field configuration on the gravitational wave signal, the amplification of the magnetic field, and the ejected material. We investigate the dependence of the phase evolution of the gravitational wave in the post-merger on the initial magnetic field, finding that dephasing between the $(\ell=2,m=2)$ mode of the gravitational wave, and the $(2,1)$ and $(3,3)$ modes may be strongly impacted by the numerical reconstruction scheme. The magnetic field amplification during the Kelvin-Helmholtz dominated phase may be considerably enhanced by anti-aligned fields, or suppressed by toroidal fields. The post-merger amplification of the field due to winding may be suppressed by toroidal fields, and enhanced by asymmetries or mixtures of poloidal and toroidal fields. The field strength in the ejecta may be impacted by the initial magnetic field, with configurations which lead to large amplifications and those with mixtures of poloidal and toroidal fields preferentially emitting highly magnetised material in the polar regions, showing a weaker dependence of the magnetic field on the density of the ejecta than in cases that amplify the magnetic field less. We find that the magnetic field is largely randomly oriented in the ejected material, supporting such models used to estimate thermalisation timescales of ejected material. We find that configurations which begin with an initial bitant symmetry break this symmetry uniformly, independent of the initial configuration, when evolved without an enforced symmetry. This behaviour suggests the presence of a spontaneous symmetry breaking bifurcation in the solution.