Resonant tidal excitation of superfluid neutron stars in coalescing binaries

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 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 (∼ 10 −3 − 10 −2 rad). A phase error of this magnitude is too small to be measured from a single event with the current generation of GW detectors.

Nonparametric inference of neutron star composition, equation of state, and maximum mass with GW170817

“…The EOS determines the eigenmode spectra within a NS, and therefore our posterior processes could be used to determine the exact placement and impact of linear resonant dynamical tidal effects due to f -modes and low-order gmodes during GW-driven inspirals (e.g., Refs. [71][72][73][74]). Similarly, knowledge of the r-mode spectra could inform the CFS instabilities relevant for millisecond pulsars [64,75,76], and knowledge of the p-and g-mode spectra could improve models of non-linear, non-resonant secular fluid instabilities relevant during the GW inspiral [77][78][79][80][81].…”

Prospects for Detecting Gravitational Waves at 5 Hz with Ground-Based Detectors

“…We find that if the NS spins at a rate greater than 35 Hz [110], a single LIGO-LF may detect the r-mode resonance up to a distance of 50 Mpc. Since the phase shift of the m=1 r-mode depends on the NS stratification, which is sensitive to the internal composition and the state of matter [111,112], a detection may thus place constraints on the NS equation of state from physics beyond the star's bulk properties [113]. Furthermore, the r-mode resonance provides an independent measurement of the NS spin, which may help break the spin-mass ratio degeneracy [14] and improve the accuracy in measuring the (equilibrium) tidal deformability [15,114].…”

Exploring the effective tidal deformability of neutron stars

“…The problem is also important from the physics point of view. Having quantified the level at which the matter composition enters the discussion we can compare to (for example) the role of the elastic crust [35] and superfluid components [44]. Finally, and perhaps most importantly, our discussion suggests a "new" approach to dynamical tides, putting the mode excitation in focus, not only for resonances associated with the m = 0 modes but also for the m = 0 contribution.…”