3,725 research outputs found
A stochastic template placement algorithm for gravitational wave data analysis
This paper presents an algorithm for constructing matched-filter template
banks in an arbitrary parameter space. The method places templates at random,
then removes those which are "too close" together. The properties and
optimality of stochastic template banks generated in this manner are
investigated for some simple models. The effectiveness of these template banks
for gravitational wave searches for binary inspiral waveforms is also examined.
The properties of a stochastic template bank are then compared to the
deterministically placed template banks that are currently used in
gravitational wave data analysis.Comment: 14 pages, 11 figure
Compton Scattering by Static and Moving Media I. The Transfer Equation and Its Moments
Compton scattering of photons by nonrelativistic particles is thought to play
an important role in forming the radiation spectrum of many astrophysical
systems. Here we derive the time-dependent photon kinetic equation that
describes spontaneous and induced Compton scattering as well as absorption and
emission by static and moving media, the corresponding radiative transfer
equation, and their zeroth and first moments, in both the system frame and in
the frame comoving with the medium. We show that it is necessary to use the
correct relativistic differential scattering cross section in order to obtain a
photon kinetic equation that is correct to first order in epsilon/m_e, T_e/m_e,
and V, where epsilon is the photon energy, T_e and m_e are the electron
temperature and rest mass, and V is the electron bulk velocity in units of the
speed of light. We also demonstrate that the terms in the radiative transfer
equation that are second-order in V usually should be retained, because if the
radiation energy density is sufficiently large compared to the radiation flux,
the effects of bulk Comptonization described by the terms that are second-order
in V are at least as important as the effects described by the terms that are
first-order in V, even when V is small. Our equations are valid for systems of
arbitrary optical depth and can therefore be used in both the free-streaming
and the diffusion regimes. We demonstrate that Comptonization by the electron
bulk motion occurs whether or not the radiation field is isotropic or the bulk
flow converges and that it is more important than thermal Comptonization if V^2
> 3 T_e/m_e.Comment: 31 pages, accepted for publication in The Astrophysical Journa
Energy Distribution in Melvin's Magnetic Universe
We use the energy-momentum complexes of Landau and Lifshitz and Papapetrou to
obtain the energy distribution in Melvin's magnetic universe. For this
space-time we find that these definitions of energy give the same and
convincing results. The energy distribution obtained here is the same as we
obtained earlier for the same space-time using the energy-momentum complex of
Einstein. These results uphold the usefulness of the energy-momentum complexes.Comment: 8 pages, RevTex, no figure
Parametric resonant acceleration of particles by gravitational waves
We study the resonant interaction of charged particles with a gravitational
wave propagating in the non-empty interstellar space in the presence of a
uniform magnetic field. It is found that this interaction can be cast in the
form of a parametric resonance problem which, besides the main resonance,
allows for the existence of many secondary ones. Each of them is associated
with a non-zero resonant width, depending on the amplitude of the wave and the
energy density of the interstellar plasma. Numerical estimates of the
particles' energisation and the ensuing damping of the wave are given.Comment: LaTeX file, 16 page
Gravitational Radiation from Rotational Instabilities in Compact Stellar Cores with Stiff Equations of State
We carry out 3-D numerical simulations of the dynamical instability in
rapidly rotating stars initially modeled as polytropes with n = 1.5, 1.0, and
0.5. The calculations are done with a SPH code using Newtonian gravity, and the
gravitational radiation is calculated in the quadrupole limit. All models
develop the global m=2 bar mode, with mass and angular momentum being shed from
the ends of the bar in two trailing spiral arms. The models then undergo
successive episodes of core recontraction and spiral arm ejection, with the
number of these episodes increasing as n decreases: this results in
longer-lived gravitational wave signals for stiffer models. This instability
may operate in a stellar core that has expended its nuclear fuel and is
prevented from further collapse due to centrifugal forces. The actual values of
the gravitational radiation amplitudes and frequencies depend sensitively on
the radius of the star R_{eq} at which the instability develops.Comment: 39 pages, uses Latex 2.09. To be published in the Dec. 15, 1996 issue
of Physical Review D. 21 figures (bitmapped). Originals available in
compressed Postscript format at ftp://zonker.drexel.edu/papers/bars
General Relativistic versus Newtonian: a universality in radiation hydrodynamics
We compare Newtonian and general relativistic descriptions of the stationary
accretion of self-gravitating fluids onto compact bodies. Spherical symmetry
and thin gas approximation are assumed. Luminosity depends, amongst other
factors, on the temperature and the contribution of gas to the total mass, in
both -- general relativistic () and Newtonian () -- models. We
discover a remarkable universal behaviour for transonic flows: the ratio of
respective luminosities is independent of the fractional mass of
the gas and depends on asymptotic temperature. It is close to 1 in the regime
of low asymptotic temperatures and can grow by one order of magnitude for high
temperatures. These conclusions are valid for a wide range of polytropic
equations of state.Comment: 8 pages, 4 figure
Quantum Phase Shift in Chern-Simons Modified Gravity
Using a unified approach of optical-mechanical analogy in a semiclassical
formula, we evaluate the effect of Chern-Simons modified gravity on the quantum
phase shift of de Broglie waves in neutron interferometry. The phase shift
calculated here reveals, in a single equation, a combination of effects coming
from Newtonian gravity, inertial forces, Schwarzschild and Chern-Simons
modified gravity. However the last two effects, though new, turn out to be too
tiny to be observed, and hence only of academic interest at present. The
approximations, wherever used, as well as the drawbacks of the non-dynamical
approach are clearly indicated.Comment: 16 pages, minor errors corrected. Accepted for publication in Phys.
Rev.
Gravitational Waves from the Dynamical Bar Instability in a Rapidly Rotating Star
A rapidly rotating, axisymmetric star can be dynamically unstable to an m=2
"bar" mode that transforms the star from a disk shape to an elongated bar. The
fate of such a bar-shaped star is uncertain. Some previous numerical studies
indicate that the bar is short lived, lasting for only a few bar-rotation
periods, while other studies suggest that the bar is relatively long lived.
This paper contains the results of a numerical simulation of a rapidly rotating
gamma=5/3 fluid star. The simulation shows that the bar shape is long lived:
once the bar is established, the star retains this shape for more than 10
bar-rotation periods, through the end of the simulation. The results are
consistent with the conjecture that a star will retain its bar shape
indefinitely on a dynamical time scale, as long as its rotation rate exceeds
the threshold for secular bar instability. The results are described in terms
of a low density neutron star, but can be scaled to represent, for example, a
burned-out stellar core that is prevented from complete collapse by centrifugal
forces. Estimates for the gravitational-wave signal indicate that a dynamically
unstable neutron star in our galaxy can be detected easily by the first
generation of ground based gravitational-wave detectors. The signal for an
unstable neutron star in the Virgo cluster might be seen by the planned
advanced detectors. The Newtonian/quadrupole approximation is used throughout
this work.Comment: Expanded version to be published in Phys. Rev. D: 13 pages, REVTeX,
13 figures, 9 TeX input file
Field of a Radiation Distributuion
General relativistic spherically symmetric matter field with a vanishing
stress energy scalar is analyzed. Procedure for generating exact solutions of
the field equations for such matter distributions is given. It is further
pointed out that all such type I spherically symmetric fields with distinct
eignvalues in the radial two space can be treated as a mixture of isotropic and
directed radiations. Various classes of exact solutions are given. Junction
conditions for such a matter field to the possible exterior solutions are also
discussed.Comment: Latex file, 13 pages, no figures. Accepted for publication in Phys.
Rev.
Laser Interferometric Detectors of Gravitational Waves
A laser interferometric detector of gravitational waves is studied and a
complete solution (to first order in the metric perturbation) of the coupled
Einstein-Maxwell equations with appropriate boundary conditions for the light
beams is determined. The phase shift, the light deflection and the rotation of
the polarization axis induced by gravitational waves are computed. The results
are compared with previous literature, and are shown to hold also for detectors
which are large in comparison with the gravitational wavelength.Comment: 13 pages, LaTe
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