Gravitational Radiation from Rapidly Rotating Nascent Neutron Stars

We study the secular evolution and gravitational wave signature of a newly-formed, rapidly rotating neutron star. The neutron star may arise from core collapse in a massive star or from the accretion-induced collapse of a white dwarf. After a brief dynamical phase, the nascent neutron star settles into an axisymmetric, secularly unstable equilibrium state. Gravitational radiation drives the star to a nonaxisymmetric, stationary equilibrium configuration via the bar-mode instability. The emitted quasi-periodic gravitational waves have a unique signature: the wave frequency sweeps downward from a few hundred Hertz to zero, while the wave amplitude increases from zero to a maximum and then decays back to zero. Such a wave signal could be detected by broad-band gravitational wave interferometers currently being constructed. We also characterize two other types of gravitational wave signals that could arise in principle from a rapidly rotating, secularly unstable neutron star: a high-frequency ($f\go 1000$ Hz) wave which increases the pattern-speed of the star, and a wave that actually increases the angular momentum of the star.