Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the stars' rotation

TL;DR

This study uses numerical relativity simulations to analyze how neutron star spins influence gravitational wave signals, mass ejection, and electromagnetic emissions in binary neutron star mergers, revealing significant effects on observable phenomena.

Contribution

It introduces detailed simulations incorporating neutron star spins, showing their impact on merger dynamics, gravitational waves, and electromagnetic counterparts, which was less explored in prior work.

Findings

Spin-orbit interactions affect late-inspiral-merger dynamics.

Spin influences gravitational wave phase differences up to 20 radians.

Aligned spins produce brighter electromagnetic counterparts.

Abstract

We present new (3+1) dimensional numerical relativity simulations of the binary neutron star (BNS) mergers that take into account the NS spins. We consider different spin configurations, aligned or antialigned to the orbital angular momentum, for equal and unequal mass BNS and for two equations of state. All the simulations employ quasiequilibrium circular initial data in the constant rotational velocity approach, i.e. they are consistent with Einstein equations and in hydrodynamical equilibrium. We study the NS rotation effect on the energetics, the gravitational waves (GWs) and on the possible electromagnetic (EM) emission associated to dynamical mass ejecta. For dimensionless spin magnitudes of $\chi\sim0.1$ we find that spin-orbit interactions and also spin-induced-quadrupole deformations affect the late-inspiral-merger dynamics. The latter is, however, dominated by finite-size effects. Spin (tidal) effects contribute to GW phase differences up to 5 (20) radians accumulated during the last eight orbits to merger. Similarly, after merger the collapse time of the remnant and the GW spectrogram are affected by the NSs rotation. Spin effects in dynamical ejecta are clearly observed in unequal mass systems in which mass ejection originates from the tidal tail of the companion. Consequently kilonovae and other EM counterparts are affected by spins. We find that spin aligned to the orbital angular momentum leads to brighter EM counterparts than antialigned spin with luminosities up to a factor of two higher.