Resonant oscillations and tidal heating in coalescing binary neutron stars

Abstract

Tidal interaction in a coalescing neutron star binary can resonantly excite the g-mode oscillations of the neutron star when the frequency of the tidal driving force equals the intrinsic g-mode frequencies. We study the g-mode oscillations of cold neutron stars using recent microscopic nuclear equations of state, where we determine self-consistently the sound speed and Brunt-Vaisala frequency in the nuclear liquid core. The properties of the g-modes associated with the stable stratification of the core depend sensitively on the pressure-density relation, as well as on the symmetry energy of the dense nuclear matter. The frequencies of the first ten g-modes lie approximately in the range 10-100 Hz. Resonant excitations of these g-modes during the last few minutes of the binary coalescence result in energy transfer and angular momentum transfer from the binary orbit to the neutron star. The angular momentum transfer is possible because a dynamical tidal lag develops even in the absence of fluid viscosity. Since the coupling between the g-mode and the tidal potential is weak, however, the amount of energy transfer during a resonance and the induced orbital phase error are very small. Resonant excitations of the g-modes play an important role in the tidal heating of binary neutron stars. Without the resonances, viscous dissipation is effective only when the stars are close to contact. The resonant oscillations result in dissipation at much larger orbital separation. The actual amount of tidal heating depends on the viscosity of the neutron star. Using the microscopic viscosity, we find that the binary neutron stars are heated to a temperature of ~ 108 K before they come into contact.