One-arm Spiral Instability in Hypermassive Neutron Stars Formed by Dynamical-Capture Binary Neutron Star Mergers

TL;DR

This study uses relativistic hydrodynamical simulations to demonstrate that certain hypermassive neutron stars formed after binary mergers can develop a one-arm spiral instability, affecting gravitational wave signals.

Contribution

It is the first to identify the development of the one-arm spiral instability in hypermassive neutron stars from dynamical capture mergers with small spins.

Findings

The instability develops within ~10 ms after merger.

The m=1 mode dominates the post-merger gravitational wave signal.

The instability occurs in remnants with a toroidal density distribution.

Abstract

Using general-relativistic hydrodynamical simulations, we show that merging binary neutron stars can form hypermassive neutrons stars that undergo the one-arm spiral instability. We study the particular case of a dynamical capture merger where the stars have a small spin, as may arise in globular clusters, and focus on an equal-mass scenario where the spins are aligned with the orbital angular momentum. We find that this instability develops when post-merger fluid vortices lead to the generation of a toroidal remnant - a configuration whose maximum density occurs in a ring around the center-of-mass - with high vorticity along its rotation axis. The instability quickly saturates on a timescale of $\sim 10$ ms, with the $m=1$ azimuthal density multipole mode dominating over higher modes. The instability also leaves a characteristic imprint on the post-merger gravitational wave signal that could be detectable if the instability persists in long-lived remnants.