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Magnetic field configurations in binary neutron star mergers. II. Inspiral, merger and ejecta

William Cook1, Eduardo M. Gutiérrez2,3, Sebastiano Bernuzzi1, David Radice2,3,4, Boris Daszuta1, Jacob Fields2,3, Peter Hammond2,3,5, Harshraj Bandyopadhyay3, and Maximilian Jacobi1

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Phys. Rev. D 114, 063050 – Published 24 September, 2026

DOI: https://doi.org/10.1103/1n82-rlhp

Abstract

We perform a series of simulations of magnetized binary neutron star mergers, with different initial magnetic field topologies, as well as varying equations of state, and mass ratios. In this paper, a companion paper to E. M. Gutiérrez, W. Cook, D. Radice, S. Bernuzzi, J. Fields, P. Hammond, B. Daszuta, H. Bandyopadhyay, and M. Jacobi, companion paper, Phys. Rev. D 114, 063049 (2026)., we analyze the impact of the initial field configuration on the emitted gravitational wave signal, the global amplification of the magnetic field, and the nature of the ejected material from the binary. We investigate the dependence of the phase evolution of the gravitational wave signal in the postmerger on the initial magnetic field, finding that dephasing effects between the dominant (ℓ=2,m=2) mode of the gravitational wave signal and leading subdominant modes (2, 1),(3, 3) may be strongly impacted by the choice of numerical reconstruction scheme. The magnetic field amplification at merger may be considerably enhanced during the Kelvin-Helmholtz instability dominated phase by the presence of antialigned fields between the stars, or suppressed by the presence of toroidal fields, while the postmerger amplification of the field due to winding may also be suppressed by toroidal fields, and may be enhanced by asymmetries or mixtures of poloidal and toroidal fields. The magnetic field strength in the ejecta may also be strongly impacted by the nature of the initial magnetic field configuration, with initial data configurations which lead to large amplifications and those with mixtures of poloidal and toroidal fields preferentially emitting highly magnetized material in the polar regions, while we find that the magnetic field strength in the ejecta for these configurations shows a weaker dependence on the density of the material than in those cases that amplify the magnetic field less. We investigate the orientation of the field in the ejecta, and find that the magnetic field is largely randomly oriented in the ejected material, providing supporting evidence for such models used to estimate thermalization timescales of ejected material. Further we find that configurations which begin with an initial bitant symmetry break this symmetry in a uniform manner, independent of the initial magnetic field configuration, when evolved without an enforced symmetry. This behavior suggests the presence of a spontaneous symmetry breaking bifurcation in the solution.

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Magnetic field configurations in binary neutron star mergers. I. Postmerger remnant and disk

Eduardo M. Gutiérrez, William Cook, David Radice, Sebastiano Bernuzzi, Jacob Fields, Peter Hammond, Boris Daszuta, Harshraj Bandyopadhyay, and Maximilian Jacobi
Phys. Rev. D 114, 063049 (2026)

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Phys. Rev. D 114, 063050– Published 24 September, 2026
  • Received 31 August 2025
  • Accepted 25 August 2026
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