Measuring the neutron star equation of state with gravitational wave observations

TL;DR

This study uses numerical simulations of neutron star mergers to evaluate how gravitational wave observations can accurately measure the neutron star equation of state, especially the radius and pressure near nuclear density.

Contribution

It demonstrates that analyzing late inspiral gravitational wave phase deviations provides a more accurate method to constrain neutron star properties than previous cutoff frequency approaches.

Findings

Distinguishable waveforms at 100 Mpc for realistic equations of state.

Neutron star radius can be measured to ~1 km accuracy at 100 Mpc.

Broadband observations between 500-1000 Hz effectively constrain the pressure near nuclear density.

Abstract

We report the results of a first study that uses numerical simulations to estimate the accuracy with which one can use gravitational wave observations of double neutron star inspiral to measure parameters of the neutron-star equation of state. The simulations use the evolution and initial-data codes of Shibata and Uryu to compute the last several orbits and the merger of neutron stars, with matter described by a parametrized equation of state. Previous work suggested the use of an effective cutoff frequency to place constraints on the equation of state. We find, however, that greater accuracy is obtained by measuring departures from the point-particle limit of the gravitational waveform produced during the late inspiral. As the stars approach their final plunge and merger, the gravitational wave phase accumulates more rapidly for smaller values of the neutron star compactness (the ratio of the mass of the neutron star to its radius). We estimate that realistic equations of state will lead to gravitational waveforms that are distinguishable from point particle inspirals at an effective distance (the distance to an optimally oriented and located system that would produce an equivalent waveform amplitude) of 100 Mpc or less. As Lattimer and Prakash observed, neutron-star radius is closely tied to the pressure at density not far above nuclear. Our results suggest that broadband gravitational wave observations at frequencies between 500 and 1000 Hz will constrain this pressure, and we estimate the accuracy with which it can be measured. Related first estimates of radius measurability show that the radius can be determined to an accuracy of ~1 km at 100 Mpc.