Structure of Stable Binary Neutron Star Merger Remnants: Role of Initial Spin

TL;DR

This study uses general relativistic simulations to explore how initial spin affects the structure, dynamics, gravitational wave signals, and matter ejection in binary neutron star mergers with stable remnants.

Contribution

It introduces new analysis methods for GW phase jumps and provides detailed insights into the role of initial spin on merger outcomes and observable signals.

Findings

Spin significantly alters vortex and deformation structures.

Gravitational waveforms can have phase jumps linked to quadrupole moments.

Spin influences the amount of matter ejected during mergers.

Abstract

We present general relativistic numerical simulations of binary neutron star (BNS) mergers with different initial spin configurations. We focus on models with stars of mass 1.4 M_sol each, which employ the equation of state (EOS) by Shen, Horowitz, and Teige, and which result in stable NSs as merger remnants. For comparison, we consider two irrotational equal mass (M=1.35 M_sol) and unequal mass (M=1.29,1.42 M_sol) BNS models using the APR4 EOS, which result in a supramassive merger remnant. We present visualizations of the fluid flow and temperature distribution and find a strong impact of the spin on vortex structure and nonaxisymmetric deformation. We compute the radial mass distribution and the rotation profile in the equatorial plane using recently developed measures independent of spatial gauge, revealing slowly rotating cores that can be well approximated by the cores of spherical stars. We also study the influence of the spin on the inspiral phase and the gravitational wave (GW) signal. Using a newly developed analysis method, we further show that gravitational waveforms from BNS mergers can exhibit one or more phase jumps after merger, which occur together with minima of the strain amplitude. We provide a natural explanation in terms of the remnant's quadrupole moment, and show that cancellation effects due to phase jumps can have a strong impact on the GW power spectrum. Finally, we discuss the impact of the spin on the amount of ejected matter.