Impact of magnetic field-driven anisotropies on the equation of state probed in neutron star mergers

TL;DR

This paper investigates how magnetic field-induced anisotropies affect the nuclear matter equation of state during neutron star mergers, using novel simulations that incorporate magnetic polarization effects and a dynamo model.

Contribution

It introduces a new numerical relativity simulation method that includes magnetic polarization tensors and a magnetic-field-dependent equation of state for the first time.

Findings

Pressure anisotropy corrections can exceed 10% in certain regions.

Magnetic field effects are potentially largest in the outer layers of the merger remnant.

The work demonstrates the significance of magnetic anisotropies in neutron star merger physics.

Abstract

Binary neutron star mergers can produce extreme magnetic fields, some of which can lead to strong magnetar-like remnants. While strong magnetic fields have been shown to affect the dynamics of outflows and angular momentum transport in the remnant, they can also crucially alter the properties of nuclear matter probed in the merger. In this work, we provide a first assessment of the latter, determining the strength of the pressure anisotropy caused by Landau level quantization and the anomalous magnetic moment. To this end, we perform the first numerical relativity simulation with a magnetic polarization tensor and a magnetic-field-dependent equation of state using a new algorithm we present here, which also incorporates a mean-field dynamo model to control the magnetic field strength present in the merger remnant. Our results show that -- in the most optimistic case -- corrections to the anisotropy can be in excess of $10\%$, and are potentially largest in the outer layers of the remnant. This work paves the way for a systematic investigation of these effects.