Dynamical Determination of the Innermost Stable Circular Orbit of Binary Neutron Stars

TL;DR

This paper uses full general relativity simulations to determine the innermost stable circular orbit (ISCO) of binary neutron stars, revealing how ISCO frequency depends on stellar compaction and spin, with implications for gravitational wave observations.

Contribution

It provides the first dynamical determination of the ISCO for binary neutron stars using full general relativity simulations, highlighting the dependence on compaction and spin.

Findings

ISCO frequency varies with stellar compaction.

Corotational binaries have smaller ISCO frequencies than turning-point estimates.

Irrotational binaries reach ISCO before the sequence terminates.

Abstract

We determine the innermost stable circular orbit (ISCO) of binary neutron stars (BNSs) by performing dynamical simulations in full general relativity. Evolving quasiequilibrium (QE) binaries that begin at different separations, we bracket the location of the ISCO by distinguishing stable circular orbits from unstable plunges. We study Gamma=2 polytropes of varying compactions in both corotational and irrotational equal-mass binaries. For corotatonal binaries we find an ISCO orbital angular frequency somewhat smaller than that determined by applying turning-point methods to QE initial data. For the irrotational binaries the initial data sequences terminate before reaching a turning point, but we find that the ISCO frequency is reached prior to the termination point. Our findings suggest that the ISCO frequency varies with compaction but does not depend strongly on the stellar spin. Since the observed gravitational wave signal undergoes a transition from a nearly periodic ``chirp'' to a burst at roughly twice the ISCO frequency, the measurement of its value by laser interferometers (e.g LIGO) will be important for determining some of the physical properites of the underlying stars