Observing and measuring the neutron-star equation-of-state in spinning binary neutron star systems

TL;DR

This paper investigates how the spin of neutron stars in binary systems affects the gravitational-wave signals and the potential to measure the neutron-star equation-of-state, highlighting that high spins may improve measurement prospects.

Contribution

It demonstrates that neutron star spin-quadrupole effects are significant and that considering high-spin systems can enhance the ability to measure the equation-of-state from gravitational waves.

Findings

Spin-quadrupole effects dominate in rapidly rotating neutron stars.

Neglecting spin-quadrupole effects has minimal impact on observation counts.

Incorrect equation-of-state assumptions can bias mass and spin measurements.

Abstract

LIGO and Virgo recently observed the first binary neutron star merger, demonstrating that gravitational-waves offer the ability to probe how matter behaves in one of the most extreme environments in the Universe. However, the gravitational-wave signal emitted by an inspiraling binary neutron star system is only weakly dependent on the equation of state and extracting this information is challenging. Previous studies have focused mainly on binary systems where the neutron stars are spinning slowly and the main imprint of neutron star matter in the inspiral signal is due to tidal effects. For binaries with non-negligible neutron-star spin the deformation of the neutron star due to its own rotation introduces additional variations in the emitted gravitational-wave signal. Here we explore whether highly spinning binary neutron-star systems offer a better chance to measure the equation-of-state than weakly spinning binary-neutron star systems. We focus on the dominant adiabatic quadrupolar effects and consider three main questions. First, we show that equation-of-state effects can be significant in the inspiral waveforms, and that the spin-quadrupole effect dominates for rapidly rotating neutron stars. Second, we show that variations in the spin-quadrupole phasing are strongly degenerate with changes in the component masses and spins, and neglecting these terms has a negligible impact on the number of observations with second generation observatories. Finally, we explore the bias in the masses and spins that would be introduced by using incorrect equation-of-state terms. Using a novel method to rapidly evaluate an approximation of the likelihood we show that assuming the incorrect equation-of-state when measuring source parameters can lead to a significant bias. We also find that the ability to measure the equation-of-state is improved when considering spinning systems.