Post-Newtonian SPH calculations of binary neutron star coalescence. II. Binary mass ratio, equation of state, and spin dependence

TL;DR

This study uses a new Post-Newtonian SPH code to analyze how neutron star binary mergers emit gravitational waves, considering effects of mass ratio, equation of state, and spin, with implications for gravitational wave detection and gamma-ray burst models.

Contribution

It introduces a detailed Post-Newtonian SPH simulation approach to explore neutron star mergers, highlighting the impact of physical parameters on gravitational wave signals.

Findings

Gravitational wave peak strain decreases steeply with lower mass ratios.

Softer equations of state produce stronger gravitational wave emission.

Irrotational initial conditions suppress gravitational wave signals but reveal a significant second peak.

Abstract

Using our new Post-Newtonian SPH (smoothed particle hydrodynamics) code, we study the final coalescence and merging of neutron star (NS) binaries. We vary the stiffness of the equation of state (EOS) as well as the initial binary mass ratio and stellar spins. Results are compared to those of Newtonian calculations, with and without the inclusion of the gravitational radiation reaction. We find a much steeper decrease in the gravity wave peak strain and luminosity with decreasing mass ratio than would be predicted by simple point-mass formulae. For NS with softer EOS (which we model as simple $\Gamma=2$ polytropes) we find a stronger gravity wave emission, with a different morphology than for stiffer EOS (modeled as $\Gamma=3$ polytropes as in our previous work). We also calculate the coalescence of NS binaries with an irrotational initial condition, and find that the gravity wave signal is relatively suppressed compared to the synchronized case, but shows a very significant second peak of emission. Mass shedding is also greatly reduced, and occurs via a different mechanism than in the synchronized case. We discuss the implications of our results for gravity wave astronomy with laser interferometers such as LIGO, and for theoretical models of gamma-ray bursts (GRBs) based on NS mergers.