1,297 research outputs found
Tethered subsatellite study
The results are presented of studies performed relating to the feasibility of deploying a subsatellite from the shuttle by means of a tether. The dynamics, the control laws, the aerodynamics, the heating, and some communication considerations of the tethered subsatellite system are considered. Nothing was found that prohibits the use of a subsatellite joined to the shuttle by a long (100 km) tether. More detailed studies directed at specific applications are recommended
Gravitational waves from axisymmetrically oscillating neutron stars in general relativistic simulations
Gravitational waves from oscillating neutron stars in axial symmetry are
studied performing numerical simulations in full general relativity. Neutron
stars are modeled by a polytropic equation of state for simplicity. A
gauge-invariant wave extraction method as well as a quadrupole formula are
adopted for computation of gravitational waves. It is found that the
gauge-invariant variables systematically contain numerical errors generated
near the outer boundaries in the present axisymmetric computation. We clarify
their origin, and illustrate it possible to eliminate the dominant part of the
systematic errors. The best corrected waveforms for oscillating and rotating
stars currently contain errors of magnitude in the local wave
zone. Comparing the waveforms obtained by the gauge-invariant technique with
those by the quadrupole formula, it is shown that the quadrupole formula yields
approximate gravitational waveforms besides a systematic underestimation of the
amplitude of where and denote the mass and the radius of
neutron stars. However, the wave phase and modulation of the amplitude can be
computed accurately. This indicates that the quadrupole formula is a useful
tool for studying gravitational waves from rotating stellar core collapse to a
neutron star in fully general relativistic simulations. Properties of the
gravitational waveforms from the oscillating and rigidly rotating neutron stars
are also addressed paying attention to the oscillation associated with
fundamental modes
Axisymmetric core collapse simulations using characteristic numerical relativity
We present results from axisymmetric stellar core collapse simulations in
general relativity. Our hydrodynamics code has proved robust and accurate
enough to allow for a detailed analysis of the global dynamics of the collapse.
Contrary to traditional approaches based on the 3+1 formulation of the
gravitational field equations, our framework uses a foliation based on a family
of outgoing light cones, emanating from a regular center, and terminating at
future null infinity. Such a coordinate system is well adapted to the study of
interesting dynamical spacetimes in relativistic astrophysics such as stellar
core collapse and neutron star formation. Perhaps most importantly this
procedure allows for the unambiguous extraction of gravitational waves at
future null infinity without any approximation, along with the commonly used
quadrupole formalism for the gravitational wave extraction. Our results
concerning the gravitational wave signals show noticeable disagreement when
those are extracted by computing the Bondi news at future null infinity on the
one hand and by using the quadrupole formula on the other hand. We have strong
indication that for our setup the quadrupole formula on the null cone does not
lead to physical gravitational wave signals. The Bondi gravitational wave
signals extracted at infinity show typical oscillation frequencies of about 0.5
kHz.Comment: 17 pages, 18 figures, submitted to Phys. Rev.
Small-x Dipole Evolution Beyond the Large-N_c Limit
We present a method to include colour-suppressed effects in the Mueller
dipole picture. The model consistently includes saturation effects both in the
evolution of dipoles and in the interactions of dipoles with a target in a
frame-independent way.
When implemented in a Monte Carlo simulation together with our previous model
of energy--momentum conservation and a simple dipole description of initial
state protons and virtual photons, the model is able to reproduce to a
satisfactory degree both the gamma*-p cross sections as measured at HERA as
well as the total p-p cross section all the way from ISR energies to the
Tevatron and beyond
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ATLAS Tracking Event Data Model
In this report the event data model (EDM) relevant for tracking in the ATLAS experiment is presented. The core component of the tracking EDM is a common track object which is suited to describe tracks in the innermost tracking sub-detectors and in the muon detectors in offline as well as online reconstruction. The design of the EDM was driven by a demand for modularity and extensibility while taking into account the different requirements of the clients. The structure of the track object and the representation of the tracking-relevant information are described in detail
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ATLAS Inner Detector Event Data Model
The data model for event reconstruction (EDM) in the Inner Detector of the ATLAS experiment is presented. Different data classes represent evolving stages in the reconstruction data flow, and specific derived classes exist for the sub-detectors. The Inner Detector EDM also extends the data model for common tracking in ATLAS and is integrated into the modular design of the ATLAS high-level trigger and off-line software
Advantages of modified ADM formulation: constraint propagation analysis of Baumgarte-Shapiro-Shibata-Nakamura system
Several numerical relativity groups are using a modified ADM formulation for
their simulations, which was developed by Nakamura et al (and widely cited as
Baumgarte-Shapiro-Shibata-Nakamura system). This so-called BSSN formulation is
shown to be more stable than the standard ADM formulation in many cases, and
there have been many attempts to explain why this re-formulation has such an
advantage. We try to explain the background mechanism of the BSSN equations by
using eigenvalue analysis of constraint propagation equations. This analysis
has been applied and has succeeded in explaining other systems in our series of
works. We derive the full set of the constraint propagation equations, and
study it in the flat background space-time. We carefully examine how the
replacements and adjustments in the equations change the propagation structure
of the constraints, i.e. whether violation of constraints (if it exists) will
decay or propagate away. We conclude that the better stability of the BSSN
system is obtained by their adjustments in the equations, and that the
combination of the adjustments is in a good balance, i.e. a lack of their
adjustments might fail to obtain the present stability. We further propose
other adjustments to the equations, which may offer more stable features than
the current BSSN equations.Comment: 10 pages, RevTeX4, added related discussion to gr-qc/0209106, the
version to appear in Phys. Rev.
Bondian frames to couple matter with radiation
A study is presented for the non linear evolution of a self gravitating
distribution of matter coupled to a massless scalar field. The characteristic
formulation for numerical relativity is used to follow the evolution by a
sequence of light cones open to the future. Bondian frames are used to endow
physical meaning to the matter variables and to the massless scalar field.
Asymptotic approaches to the origin and to infinity are achieved; at the
boundary surface interior and exterior solutions are matched guaranteeing the
Darmois--Lichnerowicz conditions. To show how the scheme works some numerical
models are discussed. We exemplify evolving scalar waves on the following fixed
backgrounds: A) an atmosphere between the boundary surface of an incompressible
mixtured fluid and infinity; B) a polytropic distribution matched to a
Schwarzschild exterior; C) a Schwarzschild- Schwarzschild spacetime. The
conservation of energy, the Newman--Penrose constant preservation and other
expected features are observed.Comment: 20 pages, 6 figures; to appear in General Relativity and Gravitatio
Tips for implementing multigrid methods on domains containing holes
As part of our development of a computer code to perform 3D `constrained
evolution' of Einstein's equations in 3+1 form, we discuss issues regarding the
efficient solution of elliptic equations on domains containing holes (i.e.,
excised regions), via the multigrid method. We consider as a test case the
Poisson equation with a nonlinear term added, as a means of illustrating the
principles involved, and move to a "real world" 3-dimensional problem which is
the solution of the conformally flat Hamiltonian constraint with Dirichlet and
Robin boundary conditions. Using our vertex-centered multigrid code, we
demonstrate globally second-order-accurate solutions of elliptic equations over
domains containing holes, in two and three spatial dimensions. Keys to the
success of this method are the choice of the restriction operator near the
holes and definition of the location of the inner boundary. In some cases (e.g.
two holes in two dimensions), more and more smoothing may be required as the
mesh spacing decreases to zero; however for the resolutions currently of
interest to many numerical relativists, it is feasible to maintain second order
convergence by concentrating smoothing (spatially) where it is needed most.
This paper, and our publicly available source code, are intended to serve as
semi-pedagogical guides for those who may wish to implement similar schemes.Comment: 18 pages, 11 figures, LaTeX. Added clarifications and references re.
scope of paper, mathematical foundations, relevance of work. Accepted for
publication in Classical & Quantum Gravit
Scalar field induced oscillations of neutron stars and gravitational collapse
We study the interaction of massless scalar fields with self-gravitating
neutron stars by means of fully dynamic numerical simulations of the
Einstein-Klein-Gordon perfect fluid system. Our investigation is restricted to
spherical symmetry and the neutron stars are approximated by relativistic
polytropes. Studying the nonlinear dynamics of isolated neutron stars is very
effectively performed within the characteristic formulation of general
relativity, in which the spacetime is foliated by a family of outgoing light
cones. We are able to compactify the entire spacetime on a computational grid
and simultaneously impose natural radiative boundary conditions and extract
accurate radiative signals. We study the transfer of energy from the scalar
field to the fluid star. We find, in particular, that depending on the
compactness of the neutron star model, the scalar wave forces the neutron star
either to oscillate in its radial modes of pulsation or to undergo
gravitational collapse to a black hole on a dynamical timescale. The radiative
signal, read off at future null infinity, shows quasi-normal oscillations
before the setting of a late time power-law tail.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
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