Constructing quasi-equilibrium initial data for binary neutron stars with arbitrary spins

TL;DR

This paper introduces a numerical method to generate initial data for binary neutron star systems with arbitrary spins, improving the modeling of realistic spinning neutron star binaries in quasi-circular orbits.

Contribution

The authors implement a new quasi-equilibrium approximation including rotational velocity for neutron star binaries, enabling more accurate initial data with arbitrary spins.

Findings

Stars with Newtonian rigid rotation velocity have near-zero expansion and shear.

Constructed initial data sequences for irrotational, corotating, and fixed rotation binaries.

Demonstrated the numerical implementation of the approximation for the first time.

Abstract

In general neutron stars in binaries are spinning. Recently, a new quasi-equilibrium approximation that includes a rotational velocity piece for each star has been proposed to describe binary neutron stars with arbitrary rotation states in quasi-circular orbits. We have implemented this approximation numerically for the first time, to generate initial data for neutron star binaries with spin. If we choose the rotational velocity piece such that it equals the Newtonian rigid rotation law, we obtain stars with fluid 4-velocities that have expansion and shear of approximately zero, as one would expect for quasi-equilibrium configurations. We also use the new approach to construct and study initial data sequences for irrotational, corotating and fixed rotation binaries.