Resonance Locking of Anharmonic $g$-Modes in Coalescing Neutron Star Binaries

Neutron stars in coalescing binaries deform due to the tidal gravitational fields generated by their companions. During the inspiral phase, the tidal deformation is dominated by the fundamental oscillation ($f$-) mode of the stars. The tide also has sub-dominant gravity ($g$-) modes that are resonantly excited when the linear tidal forcing sweeps through their eigenfrequencies. Beyond the linear order in perturbed fluid displacement, the $g$-modes are anharmonic, i.e., their oscillation frequencies depend on the mode energy. For the lowest-order $g$-mode, we show that when the tidal forcing reaches its linear eigenfrequency, the mode starts to dynamically adjust its energy so that its nonlinearly shifted oscillation frequency always matches that of the driving field. This phenomenon, which we term `resonance locking', persists through the rest of the inspiral, and hence, the mode grows to substantially larger energies than in the linear theory. Using a $1.4$--$1.4\, M_{\odot}$ binary neutron star system with the SLy4 equation of state, we find this results in an extra correction to the frequency-domain gravitational wave (GW) phase of $|\Delta \Psi|\approx 3\,{\rm rad}$ accumulated from the onset of resonance locking at the GW frequency of $94\,{\rm Hz}$ to the merger at $1.05\,{\rm kHz}$. This effect probes details of the internal structure of merging neutron stars beyond their bulk properties such as tidal deformability.