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

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 (≈15 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 quasicircular 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 ≳6.3% in the linear regime and much more as ∼33% during the later nonlinear regime. At the merger, a phase shift of ≲40  rad is rendered in the gravitational waveform as a consequence of the angular momentum and energy transfers into the neutron star oscillations.