Crustal magnetic fields do not lead to large magnetic-field amplifications in binary neutron-star mergers

TL;DR

This study uses high-resolution simulations to show that crustal magnetic fields in neutron-star mergers do not amplify as much as full-star magnetic fields, affecting astrophysical phenomena modeling.

Contribution

It demonstrates that crustal magnetic fields are less effective in amplification during mergers compared to full-star fields, highlighting the importance of magnetic topology.

Findings

Crustal magnetic fields generate strong fields during Kelvin-Helmholtz instability.

Full-star magnetic fields achieve over ten times higher amplification.

Crustal configurations lack interior magnetized material for turbulent amplification.

Abstract

The amplification of magnetic fields plays an important role in explaining numerous astrophysical phenomena associated with binary neutron-star mergers, such as mass ejection and the powering of short gamma-ray bursts. Magnetic fields in isolated neutron stars are often assumed to be confined to a small region near the stellar surface, while they are normally taken to fill the whole stars in the numerical modelling. By performing high-resolution, global, and high-order general-relativistic magnetohydrodynamic simulations we investigate the impact of a purely crustal magnetic field and contrast it with the standard configuration consisting of a dipolar magnetic field with the same magnetic energy but filling the whole star. While the crust-configurations are very effective in generating strong magnetic fields during the Kelvin-Helmholtz-instability stage, they fail to achieve the same level of magnetic-field amplification of the full-star configurations. This is due to the lack of magnetized material in the neutron-star interiors to be used for further turbulent amplification and to the surface losses of highly magnetized matter in the crust-configurations. Hence, the final magnetic energies in the two configurations differ by more than one order of magnitude. We briefly discuss the impact of these results on astrophysical observables and how they can be employed to deduce the magnetic topology in merging binaries.