Instabilities in neutron-star postmerger remnants

TL;DR

This paper uses relativistic simulations to identify and analyze a shear instability in neutron star postmerger remnants, revealing its origin near the angular velocity peak and its impact on gravitational-wave signals.

Contribution

It introduces a detailed analysis of the inner shear instability in neutron star remnants using relativistic simulations and stability theorems, linking it to observable gravitational waves.

Findings

Identified a local shear instability in neutron star remnants.

Linked the instability to a shear near the angular velocity peak.

Showed the instability evolves into a fast m=2 oscillation pattern.

Abstract

Using nonlinear, fully relativistic, simulations we investigate the dynamics and gravitational-wave signature associated with instabilities in neutron star postmerger remnants. For simplified models of the remnant we establish the presence of an instability in stars with moderate $T/|W|$, the ratio between the kinetic and the gravitational potential energies. Detailed analysis of the density oscillation pattern reveals a \emph{local} instability in the inner region of the more realistic differential rotation profile. We apply Rayleigh's inflection theorem and Fj{\o}rtoft's theorem to analyze the stability criteria concluding that this \emph{inner local} instability originates from a shear instability close to the peak of the angular velocity profile, and that it later evolves into a fast-rotating m=2 oscillation pattern. We discuss the importance of the presence of a corotation point in the fluid, its connection with the shear instability, and comparisons to the Rossby wave and Papaloizou Pringle instabilities considered in the wider literature.