Producing Magnetar Magnetic Fields in the Merger of Binary Neutron Stars

TL;DR

This paper introduces a subgrid model for simulating magnetic field amplification in binary neutron star mergers, enabling the prediction of magnetar-level fields and associated electromagnetic phenomena.

Contribution

A novel subgrid modeling approach that captures small-scale turbulence effects in global GRMHD simulations of neutron star mergers.

Findings

Magnetar-level magnetic fields (~10^{16} G) can be achieved in simulations.

Magnetic energy can reach up to ~10^{51} erg.

Post-merger remnants may emit strong electromagnetic signals.

Abstract

The merger of binary neutron stars (BNSs) can lead to large amplifications of the magnetic field due to the development of turbulence and instabilities in the fluid, such as the Kelvin-Helmholtz shear instability, which drive small-scale dynamo activity. In order to properly resolve such instabilities and obtain the correct magnetic field amplification, one would need to employ resolutions that are currently unfeasible in global general relativistic magnetohydrodynamic (GRMHD) simulations of BNS mergers. Here, we present a subgrid model that allows global simulations to take into account the small-scale amplification of the magnetic field which is caused by the development of turbulence during BNS mergers. Assuming dynamo saturation, we show that magnetar-level fields ($\sim 10^{16}\,{\rm G}$) can be easily reached, and should therefore be expected from the merger of magnetized BNSs. The total magnetic energy can reach values up to $\sim 10^{51}\,{\rm erg}$ and the post-merger remnant can therefore emit strong electromagnetic signals and possibly produce short gamma-ray bursts.