EXCITATION OF NEUTRON-STAR NORMAL-MODES DURING BINARY INSPIRAL

As a compact binary inspirals due to the emission of gravitational wav es, its orbital period decreases continuously down to approximately 1 ms, its value at coalescence. During the last part of the inspiral, th e two stars are close together, and their tidal interactions become st rong. Neutron stars have many normal modes (core g-modes, crustal disc ontinuity modes, shear modes, etc.) whose periods lie in the range (is similar to several ms) swept by the orbital period. Some of these mod es are resonantly excited by the tidal force. The amount of energy a m ode absorbs is proportional to the square of the overlap integral betw een its displacement field and the tidal force field. For all modes of interest, this overlap is poor, resulting in relatively weak excitati on. For the best case, the absorbed energy is only a small fraction (a pproximately 10(-6)) of the orbital energy, so the orbital phase shift is too weak to be detected by observations of the gravitational wave signal emitted by the inspiraling binary. However, with displacement a mplitudes of excited quadrupole modes ranging up to 0.5% of the stella r radius, the possibility of a detectable electromagnetic signature ca nnot be dismissed. Both the periods of the modes and the energy they a bsorb depend quite strongly on the internal structure of the star. The ir observation could shed light on the correct high-density equation o f state.