Structure of Stable Binary Neutron Star Merger Remnants: a Case Study

TL;DR

This paper provides a detailed analysis of the structure and dynamics of a stable neutron star merger remnant using a specific equation of state, revealing complex flow patterns, thermal features, and gravitational wave signatures.

Contribution

It introduces new diagnostic measures for quantifying merger remnant properties and offers detailed insights into the internal structure and fluid dynamics of the remnant.

Findings

Peanut-shaped fluid flow with m=2 perturbation

Locally compressive flow causes hot spots

Remnant has a slowly rotating core and outer layers near Keplerian velocity

Abstract

In this work, we study the merger of two neutron stars with a gravitational mass of 1.4 M_sol each, employing the Shen-Horowitz-Teige equation of state. This equation of state is a corner case, allowing the formation of a stable neutron star with the given total baryonic mass of 3.03 M_sol. We investigate in unprecedented detail the structure of the remnant, in particular the mass distribution, the thermal structure, and the rotation profile. We also compute fluid trajectories both inside the remnant and those relevant for the formation of the disk. We find a peanut-shaped fluid flow inside the remnant following a strong m=2 perturbation. Moreover, the flow is locally compressive, causing the appearance of dynamic hot spots. Further, we introduce new diagnostic measures that are easy to implement in numeric simulations and that allow to quantify mass and compactness of merger remnants in a well-defined way. As in previous studies of supra- and hypermassive stars, we find a remnant with a slowly rotating core and an outer envelope rotating at nearly Keplerian velocity. We compute a Tolman-Oppenheimer-Volkoff star model which agrees well with that of the remnant in the core, while the latter possesses extensive outer layers rotating close to Kepler velocity. Finally, we extract the gravitational wave signal and discuss the detectability with modern observatories. This study has implications for the interpretation of gravitational wave detections from the post-merger phase, and is relevant for short gamma-ray burst models.