Equation of state effects and one-arm spiral instability in hypermassive neutron stars formed in eccentric neutron star mergers

TL;DR

This study uses full general relativity hydrodynamic simulations to show that the one-arm spiral instability in hypermassive neutron stars from eccentric mergers is common and can inform neutron star equations of state through gravitational wave observations.

Contribution

It demonstrates the universality of the one-arm instability in hypermassive neutron stars and links gravitational wave signatures to the neutron star equation of state.

Findings

One-arm instability develops within tens of milliseconds after merger.

Stiffer equations of state produce stronger m=2 modes and weaker m=1 modes.

Detectable gravitational wave signals from the one-arm mode could constrain neutron star properties.

Abstract

We continue our investigations of the development and importance of the one-arm spiral instability in long-lived hypermassive neutron stars (HMNSs) formed in dynamical capture binary neutron star mergers. Employing hydrodynamic simulations in full general relativity, we find that the one-arm instability is generic in that it can develop in HMNSs within a few tens of milliseconds after merger for all equations of state in our survey. We find that mergers with stiffer equations of state tend to produce HMNSs with stronger $m=2$ azimuthal mode density deformations, and weaker $m=1$ components, relative to softer equations of state. We also find that for equations of state that can give rise to double-core HMNSs, large $m=1$ density modes can already be present due to asymmetries in the two cores. This results in the generation of $l=2$, $m=1$ gravitational wave modes even before the dominance of a one-arm mode that ultimately arises following merger of the two cores. Our results further suggest that stiffer equations of state give rise to HMNSs generating lower $m=1$ gravitational wave frequencies. Thus, if gravitational waves from the one-arm instability are detected, they could in principle constrain the neutron star equation of state. We estimate that, depending on the equation of state, the one-arm mode could potentially be detectable by second generation gravitational wave detectors at $\sim 10$ Mpc and by third generation ones at $\sim 100$ Mpc. Finally, we provide estimates of the properties of dynamical ejecta, as well as the accompanying kilonovae signatures.