Numerical Inside View of Hypermassive Remnant Models for GW170817

TL;DR

This study uses hydrodynamical simulations to analyze the structure and evolution of the GW170817 neutron star merger remnant, revealing insights into collapse mechanisms, gravitational wave features, and internal fluid dynamics.

Contribution

It provides a detailed 3D analysis of the post-merger remnant, highlighting the dominant role of gravitational wave emission in collapse and introducing new visualization techniques.

Findings

Remnant cannot be modeled as a simple axisymmetric neutron star.

Gravitational wave emission primarily drives collapse.

Medium-scale perturbations dominate the vorticity field inside the disk.

Abstract

The first multimessenger observation attributed to a merging neutron star binary provided an enormous amount of observational data. Unlocking the full potential of this data requires a better understanding of the merger process and the early post-merger phase, which are crucial for the later evolution that eventually leads to observable counterparts. In this work, we perform standard hydrodynamical numerical simulations of a system compatible with GW170817. We focus on a single equation of state (EOS) and two mass ratios, while neglecting magnetic fields and neutrino radiation. We then apply newly developed postprocessing and visualization techniques to the results obtained for this basic setting. The focus lies on understanding the three-dimensional structure of the remnant, most notably the fluid flow pattern, and its evolution until collapse. We investigate the evolution of mass and angular momentum distribution up to collapse, as well as the differential rotation along and perpendicular to the equatorial plane. For the cases that we studied, the remnant cannot be adequately modeled as a differentially rotating axisymetric NS. Further, the dominant aspect leading to collapse is the GW radiation and not internal redistribution of angular momentum. We relate features of the gravitational wave signal to the evolution of the merger remnant, and make the waveforms publicly available. Finally, we find that the three-dimensional vorticity field inside the disk is dominated by medium-scale perturbances and not the orbital velocity, with potential consequences for magnetic field amplification effects.