The final phase of inspiral of neutron stars: realistic equations of state

TL;DR

This paper models the final inspiral phase of neutron star and strange quark star binaries using realistic equations of state, predicting gravitational wave frequencies at the innermost stable orbit.

Contribution

It provides detailed calculations of the last inspiral stage with various equations of state, highlighting differences in the ISCO and gravitational wave signatures.

Findings

ISCO is due to orbital instability for strange quark stars.

Mass-shedding limit determines ISCO for neutron stars.

Gravitational wave frequency at ISCO ranges from 800 to 1460 Hz.

Abstract

Coalescing compact star binaries are expected to be among the strongest sources of gravitational radiation to be seen by laser interferometers. We present calculations of the final phase of inspiral of equal mass irrotational neutron star binaries and strange quark star binaries. Six types of equations of state at zero temperature are used - three realistic nuclear equations of state of various softness and three different MIT bag models of strange quark matter. We study the precoalescing stage within the Isenberg-Wilson-Mathews approximation of general relativity using a multidomain spectral method. The gravitational-radiation driven evolution of the binary system is approximated by a sequence of quasi-equilibrium configurations at fixed baryon number and decreasing separation. We find that the innermost stable circular orbit (ISCO) is given by an orbital instability for binary strange quark stars and by the mass-shedding limit for neutron star binaries. The gravitational wave frequency at the ISCO, which marks the end of the inspiral phase, is found to be around 1100-1460 Hz for two 1.35 solar masses irrotational strange stars described by the MIT bag model and between 800 Hz and 1230 Hz for neutron stars.