Resonant tidal excitation of superfluid neutron stars in coalescing binaries

TL;DR

This study examines how superfluidity in neutron star cores affects resonant tidal excitation of g modes during binary inspiral and finds that superfluidity leads to more g modes being excited, but the impact on gravitational wave phase is minimal.

Contribution

It introduces a model accounting for superfluidity in neutron star cores, revealing increased g mode excitation and similar GW phase errors compared to normal fluid models.

Findings

Superfluid cores support a denser spectrum of g modes above 10 Hz.

More orbital energy (~10x) is transferred into g modes in superfluid NSs.

GW phase error remains too small to detect with current detectors.

Abstract

We study the resonant tidal excitation of g modes in coalescing superfluid neutron star (NS) binaries and investigate how such tidal driving impacts the gravitational-wave (GW) signal of the inspiral. Previous studies of this type treated the NS core as a normal fluid and thus did not account for its expected superfluidity. The source of buoyancy that supports the g modes is fundamentally different in the two cases: in a normal fluid core the buoyancy is due to gradients in the proton-to-neutron fraction whereas in a superfluid core it is due to gradients in the muon-to-electron fraction. The latter yields a stronger stratification and a superfluid NS therefore has a denser spectrum of g modes with frequencies above 10 Hz. As a result, many more g modes undergo resonant tidal excitation as the binary sweeps through the bandwidth of GW detectors such as LIGO. We find that roughly 10 times more orbital energy is transferred into g mode oscillations if the NS has a superfluid core rather than a normal fluid core. However, because this energy is transferred later in the inspiral when the orbital decay is faster, the accumulated phase error in the gravitational waveform is comparable for a superfluid and a normal fluid NS (about 0.001-0.01 rad). A phase error of this magnitude is too small to be measured from a single event with the current generation of GW detectors.