Long-term evolution and gravitational wave radiation of neutron stars with differential rotation induced by r-modes

TL;DR

This paper investigates how r-mode-induced differential rotation affects the long-term evolution of neutron stars, revealing prolonged saturation states and persistent gravitational wave emissions that enhance detectability.

Contribution

It introduces a detailed analysis of the impact of differential rotation on neutron star evolution and gravitational wave signals, extending previous models with new long-term predictions.

Findings

Differential rotation prolongs r-mode saturation in neutron stars.

Neutron stars can emit nearly constant gravitational waves over centuries.

Enhanced gravitational wave detectability from old and nascent neutron stars.

Abstract

In a second-order r-mode theory, S'a & Tom'e found that the r-mode oscillation in neutron stars (NSs) could induce stellar differential rotation, which leads to a saturation state of the oscillation spontaneously. Based on a consideration of the coupling of the r-modes and the stellar spin and thermal evolutions, we carefully investigate the influences of the r-mode-induced differential rotation on the long-term evolutions of isolated NSs and NSs in low-mass X-ray binaries, where the viscous damping of the r-modes and its resultant effects are taken into account. The numerical results show that, for both kinds of NSs, the differential rotation can prolong the duration of the r-mode saturation state significantly. As a result, the stars can keep nearly constant temperature and angular velocity over a thousand years. Moreover, due to the long-term steady rotation of the stars, persistent quasi-monochromatic gravitational wave radiation could be expected, which increases the detectibility of gravitational waves from both nascent and accreting old NSs.