746 research outputs found
Horizon Pretracking
We introduce horizon pretracking as a method for analysing numerically
generated spacetimes of merging black holes. Pretracking consists of following
certain modified constant expansion surfaces during a simulation before a
common apparent horizon has formed. The tracked surfaces exist at all times,
and are defined so as to include the common apparent horizon if it exists. The
method provides a way for finding this common apparent horizon in an efficient
and reliable manner at the earliest possible time. We can distinguish inner and
outer horizons by examining the distortion of the surface. Properties of the
pretracking surface such as its expansion, location, shape, area, and angular
momentum can also be used to predict when a common apparent horizon will
appear, and its characteristics. The latter could also be used to feed back
into the simulation by adapting e.g. boundary or gauge conditions even before
the common apparent horizon has formed.Comment: 14 pages, 8 figures, minor change
Binary Black Holes: Spin Dynamics and Gravitational Recoil
We present a study of spinning black hole binaries focusing on the spin
dynamics of the individual black holes as well as on the gravitational recoil
acquired by the black hole produced by the merger. We consider two series of
initial spin orientations away from the binary orbital plane. In one of the
series, the spins are anti-aligned; for the second series, one of the spins
points away from the binary along the line separating the black holes. We find
a remarkable agreement between the spin dynamics predicted at 2nd
post-Newtonian order and those from numerical relativity. For each
configuration, we compute the kick of the final black hole. We use the kick
estimates from the series with anti-aligned spins to fit the parameters in the
\KKF{,} and verify that the recoil along the direction of the orbital angular
momentum is and on the orbital plane ,
with the angle between the spin directions and the orbital angular
momentum. We also find that the black hole spins can be well estimated by
evaluating the isolated horizon spin on spheres of constant coordinate radius.Comment: 15 pages, 10 figures, replaced with version accepted for publication
in PR
Gravitational recoil from spinning binary black hole mergers
The inspiral and merger of binary black holes will likely involve black holes
with both unequal masses and arbitrary spins. The gravitational radiation
emitted by these binaries will carry angular as well as linear momentum. A net
flux of emitted linear momentum implies that the black hole produced by the
merger will experience a recoil or kick. Previous studies have focused on the
recoil velocity from unequal mass, non-spinning binaries. We present results
from simulations of equal mass but spinning black hole binaries and show how a
significant gravitational recoil can also be obtained in these situations. We
consider the case of black holes with opposite spins of magnitude
aligned/anti-aligned with the orbital angular momentum, with the
dimensionless spin parameters of the individual holes. For the initial setups
under consideration, we find a recoil velocity of V = 475 \KMS a.
Supermassive black hole mergers producing kicks of this magnitude could result
in the ejection from the cores of dwarf galaxies of the final hole produced by
the collision.Comment: 8 pages, 8 figures, replaced with version accepted for publication in
Ap
Continuous Damage Fiber Bundle Model for Strongly Disordered Materials
We present an extension of the continuous damage fiber bundle model to
describe the gradual degradation of highly heterogeneous materials under an
increasing external load. Breaking of a fiber in the model is preceded by a
sequence of partial failure events occurring at random threshold values. In
order to capture the subsequent propagation and arrest of cracks, furthermore,
the disorder of the number of degradation steps of material constituents, the
failure thresholds of single fibers are sorted into ascending order and their
total number is a Poissonian distributed random variable over the fibers.
Analytical and numerical calculations showed that the failure process of the
system is governed by extreme value statistics, which has a substantial effect
on the macroscopic constitutive behaviour and on the microscopic bursting
activity as well.Comment: 10 pages, 13 figure
Percolation with excluded small clusters and Coulomb blockade in a granular system
We consider dc-conductivity of a mixture of small conducting and
insulating grains slightly below the percolation threshold, where finite
clusters of conducting grains are characterized by a wide spectrum of sizes.
The charge transport is controlled by tunneling of carriers between neighboring
conducting clusters via short ``links'' consisting of one insulating grain.
Upon lowering temperature small clusters (up to some -dependent size) become
Coulomb blockaded, and are avoided, if possible, by relevant hopping paths. We
introduce a relevant percolational problem of next-nearest-neighbors (NNN)
conductivity with excluded small clusters and demonstrate (both numerically and
analytically) that decreases as power law of the size of excluded
clusters. As a physical consequence, the conductivity is a power-law function
of temperature in a wide intermediate temperature range. We express the
corresponding index through known critical indices of the percolation theory
and confirm this relation numerically.Comment: 7 pages, 6 figure
Unequal Mass Binary Black Hole Plunges and Gravitational Recoil
We present results from fully nonlinear simulations of unequal mass binary
black holes plunging from close separations well inside the innermost stable
circular orbit with mass ratios q = M_1/M_2 = {1,0.85,0.78,0.55,0.32}, or
equivalently, with reduced mass parameters . For each case, the initial binary orbital
parameters are chosen from the Cook-Baumgarte equal-mass ISCO configuration. We
show waveforms of the dominant l=2,3 modes and compute estimates of energy and
angular momentum radiated. For the plunges from the close separations
considered, we measure kick velocities from gravitational radiation recoil in
the range 25-82 km/s. Due to the initial close separations our kick velocity
estimates should be understood as a lower bound. The close configurations
considered are also likely to contain significant eccentricities influencing
the recoil velocity.Comment: 12 pages, 5 figures, to appear in "New Frontiers" special issue of
CQ
Superkicks in Hyperbolic Encounters of Binary Black Holes
Generic inspirals and mergers of binary black holes produce beamed emission
of gravitational radiation that can lead to a gravitational recoil or kick of
the final black hole. The kick velocity depends on the mass ratio and spins of
the binary as well as on the dynamics of the binary configuration. Studies have
focused so far on the most astrophysically relevant configuration of
quasi-circular inspirals, for which kicks as large as 3,300 km/s have been
found. We present the first study of gravitational recoil in hyperbolic
encounters. Contrary to quasi-circular configurations, in which the beamed
radiation tends to average during the inspiral, radiation from hyperbolic
encounters is plunge dominated, resulting in an enhancement of preferential
beaming. As a consequence, it is possible to achieve kick velocities as large
as 10,000 km/s.Comment: 4 pages, 5 figures, 1 tabl
Numerical simulations with a first order BSSN formulation of Einstein's field equations
We present a new fully first order strongly hyperbolic representation of the
BSSN formulation of Einstein's equations with optional constraint damping
terms. We describe the characteristic fields of the system, discuss its
hyperbolicity properties, and present two numerical implementations and
simulations: one using finite differences, adaptive mesh refinement and in
particular binary black holes, and another one using the discontinuous Galerkin
method in spherical symmetry. The results of this paper constitute a first step
in an effort to combine the robustness of BSSN evolutions with very high
accuracy numerical techniques, such as spectral collocation multi-domain or
discontinuous Galerkin methods.Comment: To appear in Physical Review
Reduced basis catalogs for gravitational wave templates
We introduce a reduced basis approach as a new paradigm for modeling,
representing and searching for gravitational waves. We construct waveform
catalogs for non-spinning compact binary coalescences, and we find that for
accuracies of 99% and 99.999% the method generates a factor of about
fewer templates than standard placement methods. The continuum of gravitational
waves can be represented by a finite and comparatively compact basis. The
method is robust under variations in the noise of detectors, implying that only
a single catalog needs to be generated.Comment: Minor changes in some of the phrasing to match the version as
published in PR
Binary black hole evolutions of approximate puncture initial data
Approximate solutions to the Einstein field equations are a valuable tool to
investigate gravitational phenomena. An important aspect of any approximation
is to investigate and quantify its regime of validity. We present a study that
evaluates the effects that approximate puncture initial data, based on
"skeleton" solutions to the Einstein constraints as proposed by Faye et al.
[PRD 69, 124029 (2004)], have on numerical evolutions. Using data analysis
tools, we assess the effectiveness of these constraint-violating initial data
and show that the matches of waveforms from skeleton data with the
corresponding waveforms from constraint-satisfying initial data are > 0.97 when
the total mass of the binary is > 40M(solar). In addition, we demonstrate that
the differences between the skeleton and the constraint-satisfying initial data
evolutions, and thus waveforms, are due to negative Hamiltonian constraint
violations present in the skeleton initial data located in the vicinity of the
punctures. During the evolution, the skeleton data develops both Hamiltonian
and momentum constraint violations that decay with time, with the binary system
relaxing to a constraint-satisfying solution with black holes of smaller mass
and thus different dynamics
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