Oscillations of hypermassive compact stars with gravitational radiation and viscosity

TL;DR

This paper investigates the oscillation modes of hypermassive neutron stars formed after mergers, focusing on gravitational wave-driven instabilities and the impact of shear viscosity, with implications for transient post-merger signals.

Contribution

It models hypermassive neutron star oscillations as triaxial Riemann ellipsoids including gravitational radiation and viscosity effects, providing new insights into their stability and evolution.

Findings

Unstable modes have timescales > 1 ms relevant to merger remnants.

Gravitational wave radiation dominates secular instabilities.

Shear viscosity can stabilize modes if sufficiently large.

Abstract

Binary neutron star mergers, such as the multimessenger GW170817 event, may produce hypermassive compact objects which are supported against collapse by the internal circulation of the fluid within the star. We compute their unstable modes of oscillations driven by gravitational wave radiation and shear viscosity, modeling them as triaxial Riemann ellipsoids. We work in a perturbative regime, where the gravitational radiation reaction force is taken into account at 2.5-post-Newtonian order and find unstable modes with dissipation timescales $\gtrsim 1$ ms which are relevant to the transient state of a hypermassive remnant of a merger. We show that the secular instabilities are dominated by gravitational wave radiation. If the shear viscosity is included, it can increase the growth times or even stabilize the unstable modes, but it must have values several orders of magnitude larger than predicted for cold neutron stars.