Tayler-Spruit dynamo in binary neutron star merger remnants

¹ Max Planck Institute for Gravitational Physics (Albert Einstein Institute), D-14476 Potsdam, Germany
² Observatoire de Genève, Université de Genève, 51 Ch. Pegasi, 1290 Versoix, Switzerland
³ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁴ Université Paris Cité, Université Paris-Saclay, CNRS, CEA, AIM, F-91191 Gif-sur-Yvette, France
⁵ Center for Gravitational Physics and Quantum Information, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan
⁶ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
⁷ Astronomical Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

^⋆ Corresponding author: alexis.reboul-salze@aei.mpg.de

Received: 22 November 2024
Accepted: 9 April 2025

Abstract

Context. In binary neutron star mergers, the remnant can be stabilized by differential rotation before it collapses into a black hole. Therefore, the angular momentum transport mechanisms are crucial for predicting the lifetime of the hypermassive neutron star. One such mechanism is the Tayler-Spruit dynamo, and recent simulations have shown that it could grow in proto-neutron stars that formed during supernova explosions.

Aims. We aim to investigate whether hypermassive neutron stars with high neutrino viscosity could be unstable to the Tayler-Spruit dynamo and study how magnetic fields would evolve in this context.

Methods. Using a one-zone model based on the result of a 3D GRMHD simulation, we investigate the time evolution of the magnetic fields generated by the Tayler-Spruit dynamo. In addition, we analyze the dynamics of the 3D GRMHD simulation to determine whether the dynamo is present.

Results. Our one-zone model predicts that the Tayler-Spruit dynamo can increase the toroidal magnetic field to ≥10¹⁷ G and the dipole field to amplitudes ≥10¹⁶ G. The dynamo’s growth timescale depends on the initial large-scale magnetic field right after the merger. In the case of a long-lived hypermassive neutron star, an initial magnetic field of ≥10¹² G would be enough for the magnetic field to be amplified in a few seconds. However, we show that the resolution of the current GRMHD simulations is insufficient to resolve the Tayler-Spruit dynamo due to high numerical dissipation at small scales.

Conclusions. We find that the Tayler-Spruit dynamo could occur in hypermassive neutron stars and shorten their lifetime, which would have consequences on multi-messenger observations.