Gravitational Waves from the Merger of Binary Neutron Stars in a Fully General Relativistic Simulation

TL;DR

This paper presents high-precision 3D numerical simulations of binary neutron star mergers in full general relativity, revealing characteristic gravitational wave features depending on the merger outcome, with implications for gravitational wave astronomy.

Contribution

First to perform large-scale, high-resolution simulations of neutron star mergers in full general relativity, accurately capturing gravitational waveforms and post-merger dynamics.

Findings

Characteristic gravitational wave features depend on the merger outcome.

Long-duration quasi-periodic oscillations occur if a massive neutron star forms.

Waveforms and energy luminosity differ significantly between neutron star and black hole formation.

Abstract

We performed 3D numerical simulations of the merger of equal-mass binary neutron stars in full general relativity using a new large scale supercomputer. We take the typical grid size as (505,505,253) for (x,y,z) and the maximum grid size as (633,633,317). These grid numbers enable us to put the outer boundaries of the computational domain near the local wave zone and hence to calculate gravitational waveforms of good accuracy (within $\sim 10%$ error) for the first time. To model neutron stars, we adopt a $\Gamma$-law equation of state in the form $P=(\Gamma-1)\rho\epsilon$, where P, $\rho$, $\varep$ and $\Gamma$ are the pressure, rest mass density, specific internal energy, and adiabatic constant. It is found that gravitational waves in the merger stage have characteristic features that reflect the formed objects. In the case that a massive, transient neutron star is formed, its quasi-periodic oscillations are excited for a long duration, and this property is reflected clearly by the quasi-periodic nature of waveforms and the energy luminosity. In the case of black hole formation, the waveform and energy luminosity are likely damped after a short merger stage. However, a quasi-periodic oscillation can still be seen for a certain duration, because an oscillating transient massive object is formed during the merger. This duration depends strongly on the initial compactness of neutron stars and is reflected in the Fourier spectrum of gravitational waves. To confirm our results and to calibrate the accuracy of gravitational waveforms, we carried out a wide variety of test simulations, changing the resolution and size of the computational domain.