Evolution of the f-mode instability in neutron stars and gravitational wave detectability

April 2013

Andrea Passamonti, Erich Gaertig, Kostas D. Kokkotas, Daniela Doneva

A nascent neutron star, if rapidly rotating, may develop non-axisymmetric instabilities and radiate a significant amount of gravitational waves. One of the possible processes that can drive unstable a nonaxisymmetric mode is the well-known Chandrasekhar-Friedman-Schutz (CFS) mechanism. This instability operates when a mode which is counterrotating with respect to the star is seen as corotating by an inertial observer. The angular momentum which is then radiated by gravitational waves induces an increasingly negative angular momentum of the mode which becomes unstable.

The mode's amplification expected during the instability may generate a significant gravitational wave signal which can be potentially observed by the current and next generation of Earth-based laser interferometers. When the gravitational waves will be observed, the identification of the oscillation modes from the spectrum will help us to unveil the properties of the dense matter at super-nuclear densities and clarify the neutron star physics.

In this work we present the first dynamical study of the f-mode instability which considers relativistic rapidly rotating stars and incorporates the effect of viscosity, magnetic fields, and unstable r-modes. These are the most dominant effects which may have a significant impact on the evolution of this mode.

Considering several neutron star's models with polytropic equation of state we find that the gravitational-wave signal generated by an unstable f-mode may be potentially detected with the Einstein telescope (ET) from a source located in the Virgo cluster (20Mpc). For the more massive stellar model the signal may be even detected by the Advanced LIGO/Virgo detectors. Moreover, we found that the heat generated by shear viscosity during the saturation phase of an unstable f-mode may counterbalance the neutrino cooling and prevents the star from having superfluid transitions during the instability. The star then spends more time in the instability region and loses more angular momentum via the emission of gravitational waves.

Phys. Rev D 87, 084010 (2013) arXiv:1209.5308v1 [astro-ph.SR]