135 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
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
Improved effective-one-body description of coalescing nonspinning black-hole binaries and its numerical-relativity completion
We improve the effective-one-body (EOB) description of nonspinning coalescing
black hole binaries by incorporating several recent analytical advances,
notably: (i) logarithmic contributions to the conservative dynamics; (ii)
resummed horizon-absorption contribution to the orbital angular momentum loss;
and (iii) a specific radial component of the radiation reaction force implied
by consistency with the azimuthal one. We then complete this analytically
improved EOB model by comparing it to accurate numerical relativity (NR)
simulations performed by the Caltech-Cornell-CITA group for mass ratios
. In particular, the comparison to NR data allows us to
determine with high-accuracy () the value of the main EOB radial
potential: , where is the inter-body gravitational
potential and is the symmetric mass ratio. We introduce a new
technique for extracting from NR data an intrinsic measure of the phase
evolution, ( diagnostics). Aligning the NR-completed EOB
quadrupolar waveform and the NR one at low frequencies, we find that they keep
agreeing (in phase and amplitude) within the NR uncertainties throughout the
evolution for all mass ratios considered. We also find good agreement for
several subdominant multipoles without having to introduce and tune any extra
parameters.Comment: 42 pages, 22 figures. Improved version, to appear in Phys. Rev. D.
The EOB code will be freely available at eob.ihes.f
Binary black hole coalescence in the extreme-mass-ratio limit: Testing and improving the effective-one-body multipolar waveform
We discuss the properties of the effective-one-body (EOB) multipolar gravitational waveform emitted by nonspinning black-hole binaries of masses and M in the extreme-mass-ratio limit µ/M = v « 1. We focus on the transition from quasicircular inspiral to plunge, merger, and ringdown. We compare the EOB waveform to a Regge-Wheeler-Zerilli waveform computed using the hyperboloidal layer method and extracted at null infinity. Because the EOB waveform keeps track analytically of most phase differences in the early inspiral, we do not allow for any arbitrary time or phase shift between the waveforms. The dynamics of the particle, common to both wave-generation formalisms, is driven by a leading-order O(v) analytically resummed radiation reaction. The EOB and the Regge-Wheeler-Zerilli
waveforms have an initial dephasing of about 5 X 10^(-4) rad and maintain then a remarkably accurate phase coherence during the long inspiral (~33 orbits), accumulating only about -2 X 10^(-3) rad until the last stable orbit, i.e. ΔØ/Ø~-5.95 X 10^(-6). We obtain such accuracy without calibrating the analytically resummed EOB waveform to numerical data, which indicates the aptitude of the EOB waveform for studies concerning the Laser Interferometer Space Antenna. We then improve the behavior of the EOB
waveform around merger by introducing and tuning next-to-quasicircular corrections in both the gravitational wave amplitude and phase. For each multipole we tune only four next-to-quasicircular parameters by requiring compatibility between EOB and Regge-Wheeler-Zerilli waveforms at the light
ring. The resulting phase difference around the merger time is as small as ±0.015 rad, with a fractional amplitude agreement of 2.5%. This suggest that next-to-quasicircular corrections to the phase can be a useful ingredient in comparisons between EOB and numerical-relativity waveforms
Numerical solution of the 2+1 Teukolsky equation on a hyperboloidal and horizon penetrating foliation of Kerr and application to late-time decays
In this work we present a formulation of the Teukolsky equation for generic
spin perturbations on the hyperboloidal and horizon penetrating foliation of
Kerr recently proposed by Racz and Toth. An additional, spin-dependent
rescaling of the field variable can be used to achieve stable, long-term, and
accurate time-domain evolutions of generic spin perturbations. As an
application (and a severe numerical test), we investigate the late-time decays
of electromagnetic and gravitational perturbations at the horizon and future
null infinity by means of 2+1 evolutions. As initial data we consider four
combinations of (non-)stationary and (non-)compact-support initial data with a
pure spin-weighted spherical harmonic profile. We present an extensive study of
late time decays of axisymmetric perturbations. We verify the power-law decay
rates predicted analytically, together with a certain "splitting" behaviour of
the power-law exponent. We also present results for non-axisymmetric
perturbations. In particular, our approach allows to study the behaviour of the
late time decays of gravitational fields for nearly extremal and extremal black
holes. For rapid rotation we observe a very prolonged, weakly damped,
quasi-normal-mode phase. For extremal rotation the field at future null
infinity shows an oscillatory behaviour decaying as the inverse power of time,
while at the horizon it is amplified by several orders of magnitude over long
time scales. This behaviour can be understood in terms of the superradiance
cavity argument
Numerical relativity simulations of binary neutron stars
We present a new numerical relativity code designed for simulations of
compact binaries involving matter. The code is an upgrade of the BAM code to
include general relativistic hydrodynamics and implements state-of-the-art
high-resolution-shock-capturing schemes on a hierarchy of mesh refined
Cartesian grids with moving boxes. We test and validate the code in a series of
standard experiments involving single neutron star spacetimes. We present test
evolutions of quasi-equilibrium equal-mass irrotational binary neutron star
configurations in quasi-circular orbits which describe the late inspiral to
merger phases. Neutron star matter is modeled as a zero-temperature fluid;
thermal effects can be included by means of a simple ideal-gas prescription. We
analyze the impact that the use of different values of damping parameter in the
Gamma-driver shift condition has on the dynamics of the system. The use of
different reconstruction schemes and their impact in the post-merger dynamics
is investigated. We compute and characterize the gravitational radiation
emitted by the system. Self-convergence of the waves is tested, and we
consistently estimate error-bars on the numerically generated waveforms in the
inspiral phase
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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