Modeling dynamical ejecta from binary neutron star mergers and implications for electromagnetic counterparts

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.Comment: 25 pages, 11 figure

A corrected quadrature formula and applications

A straightforward 3-point quadrature formula of closed type is derived that improves on Simpson's rule. Just using the additional information of the integrand's derivative at the two endpoints we show the error is sixth order in grid spacing. Various error bounds for the quadrature formula are obtained to quantify more precisely the errors. Applications in numerical integration are given. With these error bounds, which are generally better than the usual Peano bounds, the composite formulas can be applied to integrands with lower order derivatives

Stability of general relativistic Miyamoto-Nagai galaxies

The stability of a recently proposed general relativistic model of galaxies is studied in some detail. This model is a general relativistic version of the well known Miyamoto-Nagai model that represents well a thick galactic disk. The stability of the disk is investigated under a general first order perturbation keeping the spacetime metric frozen (no gravitational radiation is taken into account). We find that the stability is associated with the thickness of the disk. We have that flat galaxies have more not-stable modes than the thick ones i.e., flat galaxies have a tendency to form more complex structures like rings, bars and spiral arms.Comment: 11 pages, 5 figures, accepted for publication in MNRA

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

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.Comment: 21 pages, 19 figure

Numerical relativity simulations of neutron star merger remnants using conservative mesh refinement

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 $M=2.7\,M_\odot$, mass-ratios $q=1$ and $q=1.16$, and four different equation of states (EOSs), and one configuration with a stiff EOS, $M=2.5M_\odot$ and $q=1.5$. 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 remnants form a hypermassive neutron star collapsing to a black hole+disk system on dynamical timescales, stiff EOSs can eventually produce a stable massive neutron star. Ejecta are mostly emitted around the orbital plane; favored by large mass ratios and softer EOS. The postmerger wave spectrum is mainly characterized by non-axisymmetric oscillations of the remnant. The stiff EOS configuration consisting of a $1.5M_\odot$ and a $1.0M_\odot$ neutron star shows a rather peculiar dynamics. During merger the companion star is very deformed; about~$\sim0.03M_\odot$ of rest-mass becomes unbound from the tidal tail due torque; and the merger remnant forms stable neutron star surrounded by a massive accretion disk $\sim0.3M_\odot$. 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. [abridged]Comment: 26 pages, 18 figure