Probing the Internal Composition of Neutron Stars with Gravitational Waves

TL;DR

This paper investigates whether gravitational wave observations from neutron star mergers can distinguish between different internal compositions and equations of state, especially regarding quark matter, hyperons, and kaon condensates, using Bayesian analysis.

Contribution

It demonstrates that second-generation gravitational wave detectors can effectively constrain certain exotic matter compositions in neutron stars, such as quark matter, but are less sensitive to hyperons and kaon condensates.

Findings

Second-generation detectors can heavily constrain quark matter equations of state.

Hybrid stars are difficult to distinguish from normal matter stars.

Detection of strange quark stars is possible with high confidence.

Abstract

Gravitational waves from neutron star binary inspirals contain information about the equation of state of supranuclear matter. In the absence of definitive experimental evidence that determines the correct equation of state, a number of diverse models that give the pressure in a neutron star as function of its density have been proposed. These models differ not only in the approximations and techniques they use to solve the many-body Schr\"odinger equation, but also in the neutron star composition they assume. We study whether gravitational wave observations of neutron star binaries in quasicircular inspirals will allow us to distinguish between equations of state of differing internal composition, thereby providing important information about the properties of extremely high density matter. We carry out a Bayesian model selection analysis, and find that second generation gravitational wave detectors can heavily constrain equations of state that contain only quark matter, but hybrid stars containing both normal and quark matter are harder to distinguish from normal matter stars. A gravitational wave detection with a signal-to-noise ratio of 30 and masses around $1.4M_{\odot}$ could either detect or rule out strange quark stars with a 20 to 1 confidence. The presence of kaon condensates or hyperons in neutron star inner cores cannot be easily confirmed. For example, for the equations of state studied in this paper, even a gravitational wave signal with a signal-to-noise ratio as high as 60 would not allow us to claim a detection of kaon condensates or hyperons with confidence greater than 5 to 1. On the other hand, if kaon condensates and hyperons do not form in neutron stars, a gravitational wave signal with similar signal-to-noise ratio would be able to constrain their existence with an 11 to 1 confidence for high-mass systems.