Binary Neutron Stars with Arbitrary Spins in Numerical Relativity

TL;DR

This paper introduces a new numerical relativity code for constructing initial data of binary neutron star systems with arbitrary spins, enabling detailed studies of their dynamics and gravitational wave signals.

Contribution

The authors develop a novel code based on Tichy's formalism that allows for arbitrary neutron star spins near breakup and accurately models their initial data in binary systems.

Findings

The code achieves near-breakup rotation rates for neutron stars.

Residual spins in irrotational cases are extremely small (~2e-4).

Spin and orbital precession match post-Newtonian predictions.

Abstract

We present a code to construct initial data for binary neutron star systems in which the stars are rotating. Our code, based on a formalism developed by Tichy, allows for arbitrary rotation axes of the neutron stars and is able to achieve rotation rates near rotational breakup. We compute the neutron star angular momentum through quasi-local angular momentum integrals. When constructing irrotational binary neutron stars, we find a very small residual dimensionless spin of $\sim 2\times 10^{-4}$. Evolutions of rotating neutron star binaries show that the magnitude of the stars' angular momentum is conserved, and that the spin- and orbit-precession of the stars is well described by post-Newtonian approximation. We demonstrate that orbital eccentricity of the binary neutron stars can be controlled to $\sim 0.1\%$. The neutron stars show quasi-normal mode oscillations at an amplitude which increases with the rotation rate of the stars.