Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the mass ratio

Abstract

We present new (3+1)D numerical relativity simulations of the binary neutron star (BNS) merger and postmerger phase. We focus on a previously inaccessible region of the binary parameter space spanning the binary's mass-ratio $q\sim1.00-1.75$ for different total masses and equations of state, and up to $q\sim2$ for a stiff BNS system. We study the mass-ratio effect on the gravitational waves (GWs) and on the possible electromagnetic emission associated to dynamical mass ejecta. We compute waveforms, spectra, and spectrograms of the GW strain including all the multipoles up to $l=4$. The mass-ratio has a specific imprint on the GW multipoles in the late-inspiral-merger signal, and it affects qualitatively the spectra of the merger remnant. The multipole effect is also studied by considering the dependency of the GW spectrograms on the source's sky location. Unequal mass BNSs produce more ejecta than equal mass systems with ejecta masses and kinetic energies depending almost linearly on $q$. We estimate luminosity peaks and light curves of macronovae events associated to the mergers using a simple approach. For $q\sim2$ the luminosity peak is delayed for several days and can be up to four times larger than for the $q=1$ cases. The macronova emission associated with the $q\sim2$ BNS is more persistent in time and could be observed for weeks instead of few days ($q=1$) in the near infrared. Finally, we estimate the flux of possible radio flares produced by the interaction of relativistic outflows with the surrounding medium. Also in this case a large $q$ can significantly enhance the emission and delay the peak luminosity. Overall, our results indicate that BNS merger with large mass ratio have EM signatures distinct from the equal mass case and more similar to black hole - neutron star binaries.