258 research outputs found

### Challenging the paradigm of singularity excision in gravitational collapse

A paradigm deeply rooted in modern numerical relativity calculations
prescribes the removal of those regions of the computational domain where a
physical singularity may develop. We here challenge this paradigm by performing
three-dimensional simulations of the collapse of uniformly rotating stars to
black holes without excision. We show that this choice, combined with suitable
gauge conditions and the use of minute numerical dissipation, improves
dramatically the long-term stability of the evolutions. In turn, this allows
for the calculation of the waveforms well beyond what previously possible,
providing information on the black-hole ringing and setting a new mark on the
present knowledge of the gravitational-wave emission from the stellar collapse
to a rotating black hole.Comment: 4 pages, 4 figures, accepted for publication on Phys. Rev. Let

### Accurate evolutions of inspiralling and magnetized neutron-stars: equal-mass binaries

By performing new, long and numerically accurate general-relativistic
simulations of magnetized, equal-mass neutron-star binaries, we investigate the
role that realistic magnetic fields may have in the evolution of these systems.
In particular, we study the evolution of the magnetic fields and show that they
can influence the survival of the hypermassive-neutron star produced at the
merger by accelerating its collapse to a black hole. We also provide evidence
that even if purely poloidal initially, the magnetic fields produced in the
tori surrounding the black hole have toroidal and poloidal components of
equivalent strength. When estimating the possibility that magnetic fields could
have an impact on the gravitational-wave signals emitted by these systems
either during the inspiral or after the merger we conclude that for realistic
magnetic-field strengths B<~1e12 G such effects could be detected, but only
marginally, by detectors such as advanced LIGO or advanced Virgo. However,
magnetically induced modifications could become detectable in the case of
small-mass binaries and with the development of gravitational-wave detectors,
such as the Einstein Telescope, with much higher sensitivities at frequencies
larger than ~2 kHz.Comment: 18 pages, 10 figures. Added two new figures (figures 1 and 7). Small
modifications to the text to match the version published on Phys. Rev.

### Accurate numerical simulations of inspiralling binary neutron stars and their comparison with effective-one-body analytical models

Binary neutron-star systems represent one of the most promising sources of
gravitational waves. In order to be able to extract important information,
notably about the equation of state of matter at nuclear density, it is
necessary to have in hands an accurate analytical model of the expected
waveforms. Following our recent work, we here analyze more in detail two
general-relativistic simulations spanning about 20 gravitational-wave cycles of
the inspiral of equal-mass binary neutron stars with different compactnesses,
and compare them with a tidal extension of the effective-one-body (EOB)
analytical model. The latter tidally extended EOB model is analytically
complete up to the 1.5 post-Newtonian level, and contains an analytically
undetermined parameter representing a higher-order amplification of tidal
effects. We find that, by calibrating this single parameter, the EOB model can
reproduce, within the numerical error, the two numerical waveforms essentially
up to the merger. By contrast, analytical models (either EOB, or Taylor-T4)
that do not incorporate such a higher-order amplification of tidal effects,
build a dephasing with respect to the numerical waveforms of several radians.Comment: 25 pages, 17 figs. Matched published versio

### Accurate evolutions of inspiralling neutron-star binaries: assessment of the truncation error

We have recently presented an investigation in full general relativity of the
dynamics and gravitational-wave emission from binary neutron stars which
inspiral and merge, producing a black hole surrounded by a torus (see
arXiv:0804.0594). We here discuss in more detail the convergence properties of
the results presented in arXiv:0804.0594 and, in particular, the deterioration
of the convergence rate at the merger and during the survival of the merged
object, when strong shocks are formed and turbulence develops. We also show
that physically reasonable and numerically convergent results obtained at
low-resolution suffer however from large truncation errors and hence are of
little physical use. We summarize our findings in an "error budget", which
includes the different sources of possible inaccuracies we have investigated
and provides a first quantitative assessment of the precision in the modelling
of compact fluid binaries.Comment: 13 pages, 5 figures. Minor changes to match published version. Added
figure 5 right pane

### Gravitational wave content and stability of uniformly, rotating, triaxial neutron stars in general relativity

Targets for ground-based gravitational wave interferometers include
continuous, quasiperiodic sources of gravitational radiation, such as isolated,
spinning neutron stars. In this work we perform evolution simulations of
uniformly rotating, triaxially deformed stars, the compressible analogues in
general relativity of incompressible, Newtonian Jacobi ellipsoids. We
investigate their stability and gravitational wave emission. We employ five
models, both normal and supramassive, and track their evolution with different
grid setups and resolutions, as well as with two different evolution codes. We
find that all models are dynamically stable and produce a strain that is
approximately one-tenth the average value of a merging binary system. We track
their secular evolution and find that all our stars evolve towards axisymmetry,
maintaining their uniform rotation, kinetic energy, and angular momentum
profiles while losing their triaxiality.Comment: 12 pages, 5 figure

### Binary neutron-star mergers with Whisky and SACRA: First quantitative comparison of results from independent general-relativistic hydrodynamics codes

We present the first quantitative comparison of two independent
general-relativistic hydrodynamics codes, the Whisky code and the SACRA code.
We compare the output of simulations starting from the same initial data and
carried out with the configuration (numerical methods, grid setup, resolution,
gauges) which for each code has been found to give consistent and sufficiently
accurate results, in particular in terms of cleanness of gravitational
waveforms. We focus on the quantities that should be conserved during the
evolution (rest mass, total mass energy, and total angular momentum) and on the
gravitational-wave amplitude and frequency. We find that the results produced
by the two codes agree at a reasonable level, with variations in the different
quantities but always at better than about 10%.Comment: Published on Phys. Rev.

### Analytic modelling of tidal effects in the relativistic inspiral of binary neutron stars

To detect the gravitational-wave (GW) signal from binary neutron stars and extract information about the equation of state of matter at nuclear density, it is necessary to match the signal with a bank of accurate templates. We present the two longest (to date) general-relativistic simulations of equal-mass binary neutron stars with different compactnesses, C=0.12 and C=0.14, and compare them with a tidal extension of the effective-one-body (EOB)model. The typical numerical phasing errors over the $\simeq 22$ GW cycles are $\Delta \phi\simeq \pm 0.24$ rad. By calibrating only one parameter (representing a higher-order amplification of tidal effects), the EOB model can reproduce, within the numerical error, the two numerical waveforms essentially up to the merger. By contrast, the third post-Newtonian Taylor-T4 approximant with leading-order tidal corrections dephases with respect to the numerical waveforms by several radians

### THC: a new high-order finite-difference high-resolution shock-capturing code for special-relativistic hydrodynamics

We present THC: a new high-order flux-vector-splitting code for Newtonian and
special-relativistic hydrodynamics designed for direct numerical simulations of
turbulent flows. Our code implements a variety of different reconstruction
algorithms, such as the popular weighted essentially non oscillatory and
monotonicity-preserving schemes, or the more specialised bandwidth-optimised
WENO scheme that has been specifically designed for the study of compressible
turbulence. We show the first systematic comparison of these schemes in
Newtonian physics as well as for special-relativistic flows. In particular we
will present the results obtained in simulations of grid-aligned and oblique
shock waves and nonlinear, large-amplitude, smooth adiabatic waves. We will
also discuss the results obtained in classical benchmarks such as the
double-Mach shock reflection test in Newtonian physics or the linear and
nonlinear development of the relativistic Kelvin-Helmholtz instability in two
and three dimensions. Finally, we study the turbulent flow induced by the
Kelvin-Helmholtz instability and we show that our code is able to obtain
well-converged velocity spectra, from which we benchmark the effective
resolution of the different schemes.Comment: Updated to match the published versio

### Accurate evolutions of unequal-mass neutron-star binaries: properties of the torus and short GRB engines

We present new results from accurate and fully general-relativistic
simulations of the coalescence of unmagnetized binary neutron stars with
various mass ratios. The evolution of the stars is followed through the
inspiral phase, the merger and prompt collapse to a black hole, up until the
appearance of a thick accretion disk, which is studied as it enters and remains
in a regime of quasi-steady accretion. Although a simple ideal-fluid equation
of state with \Gamma=2 is used, this work presents a systematic study within a
fully general relativistic framework of the properties of the resulting
black-hole--torus system produced by the merger of unequal-mass binaries. More
specifically, we show that: (1) The mass of the torus increases considerably
with the mass asymmetry and equal-mass binaries do not produce significant tori
if they have a total baryonic mass M_tot >~ 3.7 M_sun; (2) Tori with masses
M_tor ~ 0.2 M_sun are measured for binaries with M_tot ~ 3.4 M_sun and mass
ratios q ~ 0.75-0.85; (3) The mass of the torus can be estimated by the simple
expression M_tor(q, M_tot) = [c_1 (1-q) + c_2](M_max-M_tot), involving the
maximum mass for the binaries and coefficients constrained from the
simulations, and suggesting that the tori can have masses as large as M_tor ~
0.35 M_sun for M_tot ~ 2.8 M_sun and q ~ 0.75-0.85; (4) Using a novel technique
to analyze the evolution of the tori we find no evidence for the onset of
non-axisymmetric instabilities and that very little, if any, of their mass is
unbound; (5) Finally, for all the binaries considered we compute the complete
gravitational waveforms and the recoils imparted to the black holes, discussing
the prospects of detection of these sources for a number of present and future
detectors.Comment: 35 pages; small changes to match the published versio

### Dynamical non-axisymmetric instabilities in rotating relativistic stars

We present new results on dynamical instabilities in rapidly rotating
neutron-stars. In particular, using numerical simulations in full General
Relativity, we analyse the effects that the stellar compactness has on the
threshold for the onset of the dynamical bar-mode instability, as well as on
the appearance of other dynamical instabilities. By using an extrapolation
technique developed and tested in our previous study [1], we explicitly
determine the threshold for a wide range of compactnesses using four sequences
of models of constant baryonic mass comprising a total of 59 stellar models.
Our calculation of the threshold is in good agreement with the Newtonian
prediction and improves the previous post-Newtonian estimates. In addition, we
find that for stars with sufficiently large mass and compactness, the m=3
deformation is the fastest growing one. For all of the models considered, the
non-axisymmetric instability is suppressed on a dynamical timescale with an m=1
deformation dominating the final stages of the instability. These results,
together with those presented in [1], suggest that an m=1 deformation
represents a general and late-time feature of non-axisymmetric dynamical
instabilities both in full General Relativity and in Newtonian gravity.Comment: To appear on CQG, NFNR special issue. 16 pages, 5 color figures,
movies from http://www.fis.unipr.it/numrel/BarMode/ResearchBarMode.htm

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