We study equal and unequal-mass neutron star mergers by means of new numerical relativity simulations in which the general relativistic hydrodynamics solver employs an algorithm that guarantees mass conservation across the refinement levels of the computational mesh. We consider eight binary configurations with total mass M = 2.7 M , mass-ratios q = 1 and q = 1.16, and four different equations of state (EOSs), and one configuration with a stiff EOS, M = 2.5M and q = 1.5, which is one of the largest mass ratio simulated in numerical relativity to date. We focus on the post-merger dynamics and study the merger remnant, dynamical ejecta and the postmerger gravitational wave spectrum. Although most of the merger remnant are a hypermassive neutron star collapsing to a black hole+disk system on dynamical timescales, stiff EOSs can eventually produce a stable massive neutron star. During the merger process and on very short timescales, about ∼ 10 −3 − 10 −2 M of material become unbound with kinetic energies ∼ 10 50 erg. Ejecta are mostly emitted around the orbital plane; and favored by large mass ratios and softer EOS. The postmerger wave spectrum is mainly characterized by the non-axisymmetric oscillations of the remnant neutron star. The stiff EOS configuration consisting of a 1.5M and a 1.0M neutron star, simulated here for the first time, shows a rather peculiar dynamics. During merger the companion star is very deformed; about ∼ 0.03M of rest-mass becomes unbound from the tidal tail due to the torque generated by the two-core inner structure. The merger remnant is a stable neutron star surrounded by a massive accretion disk of rest-mass ∼ 0.3M . This and similar configurations might be particularly interesting for electromagnetic counterparts. Comparing results obtained with and without the conservative mesh refinement algorithm, we find that post-merger simulations can be affected by systematic errors if mass conservation is not enforced in the mesh refinement strategy. However, mass conservation also depends on grid details and on the artificial atmosphere setup; the latter are particularly significant in the computation of the dynamical ejecta.PACS numbers: 04.25. 04.30.Db, 95.30.Sf, 95.30.Lz, 97.60.Jd 98.62.Mw

Binary neutron star mergers are studied using nonlinear 3+1 numerical relativity simulations and the analytical effective-one-body (EOB) model. The EOB model predicts quasiuniversal relations between the mass-rescaled gravitational wave frequency and the binding energy at the moment of merger, and certain dimensionless binary tidal coupling constants depending on the stars Love numbers, compactnesses and the binary mass ratio. These relations are quasiuniversal in the sense that, for a given value of the tidal coupling constant, they depend significantly neither on the equation of state nor on the mass ratio, though they do depend on stars spins. The spin dependence is approximately linear for small spins aligned with the orbital angular momentum. The quasiuniversality is a property of the conservative dynamics; nontrivial relations emerge as the binary interaction becomes tidally dominated. This analytical prediction is qualitatively consistent with new, multi-orbit numerical relativity results for the relevant case of equal-mass irrotational binaries. Universal relations are thus expected to characterize neutron star mergers dynamics. In the context of gravitational wave astronomy, these universal relations may be used to constrain the neutron star equation of state using waveforms that model the merger accurately.

Abstract. In addition to the emission of gravitational waves (GWs) the coalescence and merger of two neutron stars will produce a variety of electromagnetic (EM) signals. In this work we combine a large set of numerical relativity simulations performed by different groups and we present fits for the mass, kinetic energy, and the velocities of the dynamical ejected material. Additionally, we comment on the geometry and composition of the ejecta and discuss the influence of the stars' individual rotation.The derived fits can be used to approximate the luminosity and lightcurve of the kilonovae (macronovae) and to estimate the main properties of the radio flares. This correlation between the binary parameters and the EM signals allows in case of a GW detection to approximate possible EM counterparts when first estimates of the masses are available. After a possible kilonovae observation our results could also be used to restrict the region of the parameter space which has to be covered by numerical relativity simulations.PACS numbers: 04.25. 04.30.Db, 95.30.Sf, 95.30.Lz, 97.60.Jd

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 ∼ 1.00 − 1.75 for different total masses and equations of state, and up to q ∼ 2 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 ∼ 2 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 ∼ 2 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.

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 χ ∼ 0.1 we find that both spin-orbit interactions and 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.