Tidal Resonance in Binary Neutron Star Inspirals: A High-Precision Study in Numerical Relativity

TL;DR

This study uses high-precision numerical relativity simulations to analyze tidal resonance effects in spinning neutron star binaries, revealing nonlinear resonance behavior, spin variations, and waveform phase shifts during inspiral and merger.

Contribution

First detailed numerical relativity analysis of tidal f-mode resonance in spinning neutron star binaries, including nonlinear effects and impact on gravitational wave signals.

Findings

Resonance extends through multiple orbits due to self-interaction.

Significant spin variation (~33%) caused by resonance.

Phase shift (~40 radians) in gravitational waveform at merger.

Abstract

We investigate the tidal resonance of the fundamental ($f$-)mode in spinning neutron stars, robustly tracing the onset of the excitation to its saturation, using numerical relativity for the first time. We performed long-term ($\approx15$~orbits) fully relativistic simulations of a merger of two highly and retrogradely spinning neutron stars. The resonance window of the $f$-mode is extended by self-interaction, and the nonlinear resonance continues up to the final plunging phase. We observe that the quasi-circular orbit is maintained throughout since the dissipation of orbit motion due to the resonance is coherent with that due to gravitational waves. The $f$-mode resonance causes a variation in the stellar spin of $\gtrsim6.3\%$ in the linear regime and much more as $\sim33\%$ during the later nonlinear regime. At the merger, a phase shift of $\lesssim40$~radians is rendered in the gravitational waveform as a consequence of the angular momentum and energy transfers into the neutron star oscillations.