Exploring properties of high-density matter through remnants of neutron-star mergers

TL;DR

This paper investigates how gravitational-wave signals from neutron-star merger remnants reveal properties of high-density matter, including the equation of state and potential evidence for two star families, through analysis of oscillation frequencies and spectral features.

Contribution

It provides a detailed analysis linking postmerger gravitational-wave frequencies to neutron star radii and maximum mass, and introduces an analytic model for gravitational-wave emission.

Findings

A tight relation exists between postmerger frequency and neutron star radii.

Knowledge of binary mass ratio is not critical for radius measurements.

A discontinuity in the postmerger frequency indicates two families of compact stars.

Abstract

Remnants of neutron-star mergers are essentially massive, hot, differentially rotating neutron stars, which are initially strongly oscillating. They represent a unique probe for high-density matter because the oscillations are detectable via gravitational-wave measurements and are strongly dependent on the equation of state. The impact of the equation of state is apparent in the frequency of the dominant oscillation mode of the remnant. For a fixed total binary mass a tight relation between the dominant postmerger frequency and the radii of nonrotating neutron stars exists. Inferring observationally the dominant postmerger frequency thus determines neutron star radii with high accuracy of the order of a few hundred meters. By considering symmetric and asymmetric binaries of the same chirp mass, we show that the knowledge of the binary mass ratio is not critical for this kind of radius measurements. We summarize different possibilities to deduce the maximum mass of nonrotating neutron stars. We clarify the nature of the three most prominent features of the postmerger gravitational-wave spectrum and argue that the merger remnant can be considered to be a single, isolated, self-gravitating object that can be described by concepts of asteroseismology. The understanding of the different mechanisms shaping the gravitational-wave signal yields a physically motivated analytic model of the gravitational-wave emission, which may form the basis for template-based gravitational-wave data analysis. We explore the observational consequences of a scenario of two families of compact stars including hadronic and quark stars. We find that this scenario leaves a distinctive imprint on the postmerger gravitational-wave signal. In particular, a strong discontinuity in the dominant postmerger frequency as function of the total mass will be a strong indication for two families of compact stars. (abridged)