How loud are neutron star mergers?

TL;DR

This study uses advanced simulations to analyze gravitational wave emissions from neutron star mergers, revealing significant energy release in the postmerger phase and dependencies on binary parameters and equations of state.

Contribution

First large parameter study of neutron star mergers with fully relativistic simulations including microphysics and neutrino cooling, highlighting postmerger gravitational wave energy and its dependencies.

Findings

Postmerger gravitational wave energy is about twice that of inspiral phase.

0.8-2.5% of binary mass-energy emitted at kHz frequencies postmerger.

Black hole spins after collapse are limited to ≤0.8, affecting gamma-ray burst models.

Abstract

We present results from the first large parameter study of neutron star mergers using fully general relativistic simulations with finite-temperature microphysical equations of state and neutrino cooling. We consider equal and unequal-mass binaries drawn from the galactic population and simulate each binary with three different equations of state. Our focus is on the emission of energy and angular momentum in gravitational waves in the postmerger phase. We find that the emitted gravitational-wave energy in the first $\sim$$10\,\mathrm{ms}$ of the life of the resulting hypermassive neutron star (HMNS) is about twice the energy emitted over the entire inspiral history of the binary. The total radiated energy per binary mass is comparable to or larger than that of nonspinning black hole inspiral-mergers. About $0.8-2.5\%$ of the binary mass-energy is emitted at kHz frequencies in the early HMNS evolution. We find a clear dependence of the postmerger GW emission on binary configuration and equation of state and show that it can be encoded as a broad function of the binary tidal coupling constant $\kappa^T_2$. Our results also demonstrate that the dimensionless spin of black holes resulting from subsequent HMNS collapse are limited to $\lesssim0.7-0.8$. This may significantly impact the neutrino pair annihilation mechanism for powering short gamma-ray bursts (sGRB).