Binary Neutron-Star Systems: From the Newtonian Regime to the Last Stable Orbit

TL;DR

This paper presents the first fully relativistic calculations of binary neutron-star orbits, covering from the innermost stable orbit to large separations where Newtonian physics applies, providing new insights into their orbital dynamics.

Contribution

It introduces a method to compute fully relativistic binary neutron-star orbits across a wide range of separations, including the ISCO, using a conformally flat spatial metric.

Findings

Numerical results agree with Newtonian physics at large distances.

Identified the innermost stable circular orbit (ISCO) for different stellar masses.

Provided a self-consistent determination of the ISCO.

Abstract

We report on the first calculations of fully relativistic binary circular orbits to span a range of separation distances from the innermost stable circular orbit (ISCO), deeply inside the strong field regime, to a distance ($\sim$ 200 km) where the system is accurately described by Newtonian dynamics. We consider a binary system composed of two identical corotating neutron stars, with 1.43 $M_\odot$ gravitational mass each in isolation. Using a conformally flat spatial metric we find solutions to the initial value equations that correspond to semi-stable circular orbits. At large distance, our numerical results agree exceedingly well with the Newtonian limit. We also present a self consistent determination of the ISCO for different stellar masses.