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

TL;DR

This paper discovers a gravitational-wave-driven secular instability in neutron star binaries that affects their orbital and spin evolution, potentially slowing inspiral and reducing stellar spin.

Contribution

It identifies a new instability mechanism acting on the equilibrium tide in neutron star binaries, similar to the CFS instability, and models its time scale and effects.

Findings

Instability active when primary star's spin exceeds orbital frequency.

Time scale of instability can be as short as a few seconds.

Instability influences orbital decay and stellar spin evolution.

Abstract

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.