Equation-of-state-independent relations in neutron stars

TL;DR

This paper investigates the universal relations between neutron star properties, demonstrating their robustness in dynamic scenarios and their potential for gravitational-wave-based measurements of neutron star characteristics.

Contribution

It extends the understanding of I-lambda-Q relations by showing their insensitivity to the equation of state during inspiral, and provides practical fits for gravitational-wave analysis.

Findings

I-lambda-Q relations are nearly independent of the equation of state during inspiral.

The universal relation holds up to gravitational wave frequencies of ~900 Hz.

Potential to measure neutron star radius with about 10% accuracy using gravitational waves.

Abstract

Neutron stars are extremely relativistic objects which abound in our universe and yet are poorly understood, due to the high uncertainty on how matter behaves in the extreme conditions which prevail in the stellar core. It has recently been pointed out that the moment of inertia I, the Love number lambda and the spin-induced quadrupole moment Q of an isolated neutron star, are related through functions which are practically independent of the equation of state. These surprising universal I-lambda-Q relations pave the way for a better understanding of neutron stars, most notably via gravitational-wave emission. Gravitational-wave observations will probe highly-dynamical binaries and it is important to understand whether the universality of the I-lambda-Q relations survives strong-field and finite-size effects. We apply a Post-Newtonian-Affine approach to model tidal deformations in compact binaries and show that the I-lambda relation depends on the inspiral frequency, but is insensitive to the equation of state. We provide a fit for the universal relation, which is valid up to a gravitational wave frequency of ~900 Hz and accurate to within a few percent. Our results strengthen the universality of I-lambda-Q relations, and are relevant for gravitational-wave observations with advanced ground-based interferometers. We also discuss the possibility of using the Love-compactness relation to measure the neutron-star radius with an uncertainty of about 10% or smaller from gravitational-wave observations.