Gravitational-wave-driven tidal secular instability in neutron star binaries

Understand

We report the existence of a gravitational-wave-driven secular instability in neutron star binaries, acting on the equilibrium tide.

• The instability is similar to the classic Chandrasekhar-Friedman-Schutz (CFS) instability of normal modes and is active when the spin of the primary star exceeds the orbital frequency of the companion.
• Modeling the neutron star as a Newtonian n=1 polytrope, we calculate the instability time scale, which can be as low as a few seconds at small orbital separations but still larger than the inspiral time scale.
• The implications for orbital and spin evolution are also briefly explored, where it is found that the instability slows down the inspiral and decreases the stellar spin.