Binary neutron stars: Equilibrium models beyond spatial conformal flatness

TL;DR

This paper develops a new numerical approach to model binary neutron stars in close orbits without assuming spatial conformal flatness, providing more accurate initial data for gravitational wave studies.

Contribution

It introduces a waveless formulation solving the full Einstein-relativistic-Euler system for binary neutron stars, surpassing previous conformally flat models.

Findings

Deviations from third post-Newtonian results near final orbit.

New solutions useful for merger simulations and gravitational wave templates.

Improved estimates of gravitational-wave cutoff frequency.

Abstract

Equilibria of binary neutron stars in close circular orbits are computed numerically in a waveless formulation: The full Einstein-relativistic-Euler system is solved on an initial hypersurface to obtain an asymptotically flat form of the 4-metric and an extrinsic curvature whose time derivative vanishes in a comoving frame. Two independent numerical codes are developed, and solution sequences that model inspiraling binary neutron stars during the final several orbits are successfully computed. The binding energy of the system near its final orbit deviates from earlier results of third post-Newtonian and of spatially conformally flat calculations. The new solutions may serve as initial data for merger simulations and as members of quasiequilibrium sequences to generate gravitational wave templates, and may improve estimates of the gravitational-wave cutoff frequency set by the last inspiral orbit.