Crustal magnetic fields do not lead to large magnetic-field amplifications in binary neutron-star mergers (2211.13661v2)

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.

• The paper demonstrates that crust-confined magnetic fields achieve lower amplification than full-star fields during binary neutron-star mergers.
• It uses high-resolution GRMHD simulations to capture four evolution stages, with initial rapid amplification in crustal configurations that quickly declines.
• The study highlights how turbulent processes and the shedding of magnetized material limit amplification, refining models for observable astrophysical phenomena.

Analysis of Crustal and Full-Star Magnetic Field Configurations in Neutron-Star Mergers

The paper conducted by Chabanov et al. explores the role of crustal versus full-star magnetic field configurations in binary neutron-star (BNS) mergers. Through high-resolution, general-relativistic magnetohydrodynamics (GRMHD) simulations, the paper demonstrates that largely magnetic fields confined to the crust do not achieve comparable amplification levels as fields initially permeating the entire star. This nuanced investigation addresses critical questions about magnetic field behaviors during neutron star coalescence events.

Key Findings

1. Magnetic Field Topologies and Amplification: The research contrasts two configurations — full-star and crust-confined — both initially possessing similar magnetic energy. The findings reveal that the crustal arrangement, while potent in the initial amplification stage, ultimately falls short of the amplification reached by full-configurations. This difference is attributed to a lack of magnetized material in crust-configurations available for turbulent amplification and significant losses of magnetized matter from the star surface.
2. Stages of Evolution: Both configurations experience four principal stages:
   • The Kelvin-Helmholtz-instability (KHI) driven stage, where shearing generates initial turbulence,
   • A decay stage, characterized by insufficient amplification,
   • The turbulent-amplification stage, which sees renewed magnetic growth,
   • Finally, the winding stage, characterized by shearing motion-induced amplification.

Notably, the crust-configurations amplify faster initially due to a more concentrated field, but this advantage is short-lived.

1. Resolution Impact: High-resolution simulations allow for a more refined capture of turbulence and magnetic evolution. The research indicates that high resolution enables the detection of fine structures and highlights the significance of shedding magnetized layers, which could affect the amplification potential of crustal configurations.

Numerical and Theoretical Implications

The detailed use of numerical simulations provides evidence for understanding the role of initial magnetic configurations in BNS mergers. It indicates that intense magnetic fields in the post-merger can result in different astrophysical observations, with implications on gravitational wave characteristics and gamma-ray burst (GRB) generation scenarios.

Importantly, the specification of crustal versus full-star field topology may refine models predicting GRB and kilonova production, impacting theoretical studies connecting such cosmic events to observable phenomena. Furthermore, these insights can inform subgrid-scale modeling in less-resolved simulations, enhancing the fidelity of large-scale numerical frameworks.

Implications for Further Research

This work prompts several avenues for future investigation. Research could explore a wider range of magnetic field configurations or varying density stratifications to explore potential intermediary behaviors not captured in either crustal or full-star scenarios. Additionally, coupling advanced GRMHD models with electromagnetic and gravitational wave observations may further elucidate the nature of magnetic-field driven outflows and their relation to central engine models in GRBs.

In conclusion, Chabanov et al. provide an analytical framework and simulation validation essential to understanding the dynamics of magnetic fields in neutron star interactions. It underlies the necessity of comprehensive simulation studies in assessing magnetic field impacts, enhancing the theoretical foundations supporting gravitational wave and high-energy astrophysical phenomena.