1,640 research outputs found
Simulations of rotating neutron star collapse with the puncture gauge: end state and gravitational waveforms
We reexamine the gravitational collapse of rotating neutron stars to black
holes by new 3+1 numerical relativity simulations employing the Z4c formulation
of Einstein equations, the moving puncture gauge conditions, and a conservative
mesh refinement scheme for the general relativistic hydrodynamics. The end
state of the collapse is compared to the vacuum spacetime resulting from the
evolution of spinning puncture initial data. Using a local analysis for the
metric fields, we demonstrate that the two spacetimes actually agree.
Gravitational waveforms are analyzed in some detail. We connect the emission of
radiation to the collapse dynamics using simplified spacetime diagrams, and
discuss the similarity of the waveform structure with the one of black hole
perturbation theory.Comment: 9 pages, 9 figure
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
Closed-form tidal approximants for binary neutron star gravitational waveforms constructed from high-resolution numerical relativity simulations
We construct closed-form gravitational waveforms (GWs) with tidal effects for
the coalescence and merger of binary neutron stars. The method relies on a new
set of eccentricity-reduced and high-resolution numerical relativity (NR)
simulations and is composed of three steps. First, tidal contributions to the
GW phase are extracted from the time-domain NR data. Second, those
contributions are employed to fix high-order coefficients in an effective and
resummed post-Newtonian expression. Third, frequency-domain tidal approximants
are built using the stationary phase approximation. Our tidal approximants are
valid from the low frequencies to the strong-field regime and up to merger.
They can be analytically added to any binary black hole GW model to obtain a
binary neutron star waveform, either in the time or in the frequency domain.
This work provides simple, flexible, and accurate models ready to be used in
both searches and parameter estimation of binary neutron star events
Disk formation in the collapse of supramassive neutron stars
Short gamma-ray bursts (sGRBs) show a large diversity in their properties.
This suggests that the observed phenomenon can be caused by different "central
engines" or that the engine produces a variety of outcomes depending on its
parameters, or possibly both. The most popular engine scenario, the merger of
two neutron stars, has received support from the recent Fermi and INTEGRAL
detection of a burst of gamma rays (GRB170817A) following the neutron star
merger GW170817, but at the moment it is not clear how peculiar this event
potentially was. Several sGRBs engine models involve the collapse of a
supramassive neutron star that produces a black hole plus an accretion disk. We
study this scenario for a variety of equations of states both via angular
momentum considerations based on equilibrium models and via fully dynamical
Numerical Relativity simulations. We obtain a broader range of disk forming
configurations than earlier studies but we agree with the latter that none of
these configurations is likely to produce a phenomenon that would be classified
as an sGRB.Comment: accepted by MNRA
Statistical separation of observed global and European climate data into natural and anthropogenic signals
Observed global and European spatiotemporal related fields of surface air temperature, mean-sea-level pressure and precipitation are analyzed statistically with respect to their response to external forcing factors such as anthropogenic greenhouse gases, anthropogenic sulfate aerosol, solar variations and explosive volcanism, and known internal climate mechanisms such as the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). As a first step, a principal component analysis (PCA) is applied to the observed spatiotemporal related fields to obtain spatial patterns with linear independent temporal structure. In a second step, the time series of each of the spatial patterns is subject to a stepwise regression analysis in order to separate it into signals of the external forcing factors and internal climate mechanisms as listed above as well as the residuals. Finally a back-transformation leads to the spatiotemporally related patterns of all these signals being intercompared. Two kinds of significance tests are applied to the anthropogenic signals. First, it is tested whether the anthropogenic signal is significant compared with the complete residual variance including natural variability. This test answers the question whether a significant anthropogenic climate change is visible in the observed data. As a second test the anthropogenic signal is tested with respect to the climate noise component only. This test answers the question whether the anthropogenic signal is significant among others in the observed data. Using both tests, regions can be specified where the anthropogenic influence is visible (second test) and regions where the anthropogenic influence has already significantly changed climate (first test)
Mergers of binary neutron stars with realistic spin
Simulations of binary neutron stars have seen great advances in terms of
physical detail and numerical quality. However, the spin of the neutron stars,
one of the simplest global parameters of binaries, remains mostly unstudied. We
present the first, fully nonlinear general relativistic dynamical evolutions of
the last three orbits for constraint satisfying initial data of spinning
neutron star binaries, with astrophysically realistic spins aligned and
anti-aligned to the orbital angular momentum. The initial data is computed with
the constant rotational velocity approach. The dynamics of the systems is
analyzed in terms of gauge-invariant binding energy vs. orbital angular
momentum curves. By comparing to a binary black hole configuration we can
estimate the different tidal and spin contributions to the binding energy for
the first time. First results on the gravitational wave forms are presented.
The phase evolution during the orbital motion is significantly affected by
spin-orbit interactions, leading to delayed or early mergers. Furthermore, a
frequency shift in the main emission mode of the hyper massive neutron star is
observed. Our results suggest that a detailed modeling of merger waveforms
requires the inclusion of spin, even for the moderate magnitudes observed in
binary neutron star systems
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
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
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