41 research outputs found
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
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 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 , mass-ratios and
, and four different equation of states (EOSs), and one configuration
with a stiff EOS, and . 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 and a neutron star shows a rather
peculiar dynamics. During merger the companion star is very deformed;
about~ 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 . 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
Numerical Relativity Simulations of Precessing Binary Neutron Star Mergers
We present the first set of numerical relativity simulations of binary
neutron mergers that include spin precession effects and are evolved with
multiple resolutions. Our simulations employ consistent initial data in general
relativity with different spin configurations and dimensionless spin magnitudes
. They start at a gravitational-wave frequency of ~Hz and
cover more than precession period and about 15 orbits up to merger. We
discuss the spin precession dynamics by analyzing coordinate trajectories,
quasi-local spin measurements, and energetics, by comparing spin aligned,
antialigned, and irrotational configurations. Gravitational waveforms from
different spin configuration are compared by calculating the mismatch between
pairs of waveforms in the late inspiral. We find that precession effects are
not distinguishable from nonprecessing configurations with aligned spins for
approximately face-on binaries, while the latter are distinguishable from a
nonspinning configurations. Spin precession effects are instead clearly visible
for approximately edge-on binaries.
For the parameters considered here, precession does not significantly affect
the characteristic postmerger gravitational-wave frequencies nor the mass
ejection. Our results pave the way for the modeling of spin precession effects
in the gravitational waveform from binary neutron star events.Comment: 12 pages, 10 figures, comments are welcom
Quasiuniversal properties of neutron star mergers
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
Simulating Binary Neutron Stars with Hybrid Equation of States: Gravitational Waves, Electromagnetic Signatures, and Challenges for Numerical Relativity
The gravitational wave and electromagnetic signatures connected to the merger
of two neutron stars allow us to test the nature of matter at supranuclear
densities. Since the Equation of State governing the interior of neutron stars
is only loosely constrained, there is even the possibility that strange quark
matter exists inside the core of neutron stars. We investigate how strange
quark matter cores affect the binary neutron star coalescence by performing
numerical relativity simulations. Interestingly, the strong phase transition
can cause a reduction of the convergence order of the numerical schemes to
first order if the numerical resolution is not high enough. Therefore, an
additional challenge is added in producing high-quality gravitational wave
templates for Equation of States with a strong phase transition. Focusing on
one particular configuration of an equal mass configuration consistent with
GW170817, we compute and discuss the associated gravitational wave signal and
some of the electromagnetic counterparts connected to the merger of the two
stars. We find that existing waveform approximants employed for the analysis of
GW170817 allow describing this kind of systems within the numerical
uncertainties, which, however, are several times larger than for pure hadronic
Equation of States, which means that even higher resolutions have been employed
for an accurate gravitational wave model comparison. We also show that for the
chosen Equation of State, quasi-universal relations describing the
gravitational wave emission after the moment of merger seem to hold and that
the electromagnetic signatures connected to our chosen setup would not be
bright enough to explain the kilonova associated to GW170817
The Radiant Massive Magnetic Dipole
We present an exact, time-dependent solution for the Einstein field equations
that models the coupling between an anisotropic fluid and a magnetic field in
an axially symmetric space-time. By carefully selecting the metric components,
we achieve a convenient separation of variables that enables us to solve
Einstein's field equations and obtain a solution that evolves into the
Gutsunaev-Manko massive magnetic dipole. The analysis of the thermodynamic
quantities suggests that this solution may represent a pulse of radiation
emitted by a massive object with magnetic properties as for example pulsars or
neutron stars
Towards rapid transient identification and characterization of kilonovae
With the increasing sensitivity of advanced gravitational wave detectors, the
first joint detection of an electromagnetic and gravitational wave signal from
a compact binary merger will hopefully happen within this decade. However,
current gravitational-wave likelihood sky areas span , and thus it is a challenging task to identify which,
if any, transient corresponds to the gravitational-wave event. In this study,
we make a comparison between recent kilonovae/macronovae lightcurve models for
the purpose of assessing potential lightcurve templates for counterpart
identification. We show that recent analytical and parametrized models for
these counterparts result in qualitative agreement with more complicated
radiative transfer simulations. Our analysis suggests that with improved
lightcurve models with smaller uncertainties, it will become possible to
extract information about ejecta properties and binary parameters directly from
the lightcurve measurement. Even tighter constraints are obtained in cases for
which gravitational-wave and kilonovae parameter estimation results are
combined. However, to be prepared for upcoming detections, more realistic
kilonovae models are needed. These will require numerical relativity with more
detailed microphysics, better radiative transfer simulations, and a better
understanding of the underlying nuclear physics
Potential flows in a core-dipole-shell system: numerical results
Numerical solutions for: the integral curves of the velocity field
(streamlines), the density contours, and the accretion rate of a steady-state
flow of an ideal fluid with p=K n^(gamma) equation of state orbiting in a
core-dipole-shell system are presented. For 1 < gamma < 2, we found that the
non-linear contribution appearing in the partial differential equation for the
velocity potential has little effect in the form of the streamlines and density
contour lines, but can be noticed in the density values. The study of several
cases indicates that this appears to be the general situation. The accretion
rate was found to increase when the constant gamma decreases.Comment: RevTex, 8 pages, 5 eps figures, CQG to appea