Gravitomagnetic resonant excitation of Rossby modes in coalescing neutron star binaries

TL;DR

This paper investigates how gravitomagnetic tidal forces resonantly excite Rossby modes in neutron stars during binary coalescence, affecting gravitational wave signals and inspiral dynamics.

Contribution

It introduces the gravitomagnetic excitation mechanism for Rossby modes, which dominates over previous Newtonian tidal effects, and quantifies its impact on gravitational wave phase shifts.

Findings

Gravitomagnetic driving excites specific Rossby modes in neutron stars.

The phase shift in gravitational waves can reach detectable levels for larger neutron star radii.

Energy transfer from orbit to star generally slows down the inspiral, especially for modes satisfying the CFS instability.

Abstract

In coalescing neutron star binaries, r-modes in one of the stars can be resonantly excited by the gravitomagnetic tidal field of its companion. This post-Newtonian gravitomagnetic driving of these modes dominates over the Newtonian tidal driving previously computed by Ho and Lai. To leading order in the tidal expansion parameter R/r (where R is the radius of the neutron star and r is the orbital separation), only the l=2, |m|= 1 and |m| = 2 r-modes are excited. The tidal work done on the star through this driving has an effect on the evolution of the inspiral and on the phasing of the emitted gravitational wave signal. For a neutron star of mass M, radius R, spin frequency f_spin, modeled as a Gamma =2 polytrope, with a companion also of mass M, the gravitational wave phase shift for the m=2 mode is (0.1radians)(R/10km)^4(M/1.4M_sun)^{-10/3}(f_spin/100Hz)^{2/3} for optimal spin orientation. For canonical neutron star parameters this phase shift will likely not be detectable by gravitational wave detectors such as LIGO, but if the neutron star radius is larger it may be detectable if the signal-to-noise ratio is moderately large. For neutron star - black hole binaries, the effect is smaller; the phase shift scales as companion mass to the -4/3 power for large companion masses. The net energy transfer from the orbit into the star is negative corresponding to a slowing down of the inspiral. This occurs because the interaction reduces the spin of the star, and occurs only for modes which satisfy the Chandrasekhar-Friedman-Schutz instability criterion.