27,342 research outputs found
Numerical Models of Spin-Orbital Coupling in Neutron Star Binaries
We present a new numerical scheme for solving the initial value problem for
quasiequilibrium binary neutron stars allowing for arbitrary spins. We
construct sequences of circular-orbit binaries of varying separation, keeping
the rest mass and circulation constant along each sequence. The spin angular
frequency of the stars is shown to vary along the sequence, a result that can
be derived analytically in the PPN limit. This spin effect, in addition to
leaving an imprint on the gravitational waveform emitted during binary
inspiral, is measurable in the electromagnetic signal if one of the stars is a
pulsar visible from Earth.Comment: 4 pages, 3 figures. Submitted to the Proceedings of the "X Marcel
Grossmann Meeting on General Relativity" in Rio de Janeiro, Brazil, July
20-26 (2003
Impact of Rotation on Quark-Hadron Hybrid Stars
Many recent observations give restrictions to the equation of state (EOS) for
high-density matter. Theoretical studies are needed to try to elucidate these
EOSs at high density and/or temperature. With the many known rapidly rotating
neutron stars, e.g., pulsars, several theoretical studies have tried to take
into account the effects of rotation. In our study of these systems, we find
that one of our EOSs is consistent with recent observation, whereas the other
is inconsistent.Comment: Quarks and Compact Stars 201
Critical Temperature for -Particle Condensation within a Momentum Projected Mean Field Approach
Alpha-particle (quartet) condensation in homogeneous spin-isospin symmetric
nuclear matter is investigated. The usual Thouless criterion for the critical
temperature is extended to the quartet case. The in-medium four-body problem is
strongly simplified by the use of a momentum projected mean field ansatz for
the quartet. The self-consistent single particle wave functions are shown and
discussed for various values of the density at the critical temperature
Hydrodynamics of Binary Coalescence.I. Polytropes with Stiff Equations of State
We have performed a series of three-dimensional hydrodynamic calculations of
binary coalescence using the smoothed particle hydrodynamics (SPH) method. The
initial conditions are exact polytropic equilibrium configurations with \gam >
5/3, on the verge of dynamical instability. We calculate the emission of
gravitational radiation in the quadrupole approximation. The fully nonlinear
development of the instability is followed until a new equilibrium
configuration is reached. We find that the properties of this final
configuration depend sensitively on both the compressibility and mass ratio. An
{\em axisymmetric} merged configuration is always produced when \gam\lo2.3.
As a consequence, the emission of gravitational radiation shuts off abruptly
right after the onset of dynamical instability. In contrast, {\em triaxial\/}
merged configurations are obtained when \gam\go2.3, and the system continues
to emit gravitational waves after the final coalescence. Systems with mass
ratios typically become dynamically unstable before the onset of mass
transfer. Stable mass transfer from one neutron star to another in a close
binary is therefore probably ruled out. The maximum amplitude and
peak luminosity of the gravitational waves emitted during the final
coalescence are nearly independent of \gam, but depend very sensitively on
the mass ratio .Comment: 27 pages, uuencoded compressed postscript, 16 figures upon request
from [email protected], IAS-AST-94-
The action for the (propagating) torsion and the limits on the torsion parameters from present experimental data
Starting from the well established form of the Dirac action coupled to the
electromagnetic and torsion field we find that there is some additional softly
broken local symmetry associated with torsion. This symmetry fixes the form of
divergences of the effective action after the spinor fields are integrated out.
Then the requirement of renormalizability fixes the torsion field to be
equivalent to some massive pseudovector and its action is fixed with accuracy
to the values of coupling constant of torsion-spinor interaction, mass of the
torsion and higher derivative terms. Implementing this action into the abelian
sector of the Standard Model we establish the upper bounds on the torsion mass
and coupling. In our study we used results of present experimental limits on
four-fermion contact interaction (LEP, HERA, SLAC, SLD, CCFR) and TEVATRON
limits on the cross section of new gauge boson, which could be produced as a
resonance at high energy collisions.Comment: 12 pages, LaTeX, 5 figures include
Region of the anomalous compression under Bondi-Hoyle accretion
We investigate the properties of an axisymmetric non-magnetized gas flow
without angular momentum on a small compact object, in particular, on a
Schwarzschild black hole in the supersonic region near the object; the velocity
of the object itself is assumed to be low compared to the speed of sound at
infinity. First of all, we see that the streamlines intersect (i.e., a caustic
forms) on the symmetry axis at a certain distance from the center on the
front side if the pressure gradient is neglected. The characteristic radial
size of the region, in which the streamlines emerging from the sonic surface at
an angle no larger than to the axis intersect, is To refine the flow structure in this region, we numerically
compute the system in the adiabatic approximation without ignoring the
pressure. We estimate the parameters of the inferred region with anomalously
high matter temperature and density accompanied by anomalously high energy
release.Comment: 10 pages, 2 figure
Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem
Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure
Spin-orbit correlation energy in neutron matter
We study the relevance of the energy correlation produced by the two-body
spin-orbit coupling present in realistic nucleon-nucleon potentials. To this
purpose, the neutron matter Equation of State (EoS) is calculated with the
realistic two-body Argonne potential. The shift occuring in the EoS when
spin-orbit terms are removed is taken as an estimate of the spin-orbit
correlation energy. Results obtained within the Bethe-Brueckner-Goldstone
expansion, extended up to three hole-line diagrams, are compared with other
many-body calculations recently presented in the literature. In particular,
excellent agreement is found with the Green's function Monte-Carlo method. This
agreement indicates the present theoretical accuracy in the calculation of the
neutron matter EoS.Comment: 5 pages, 2 figures, 2 tables; to appear in Phys. Rev.
An associative memory for the on-line recognition and prediction of temporal sequences
This paper presents the design of an associative memory with feedback that is
capable of on-line temporal sequence learning. A framework for on-line sequence
learning has been proposed, and different sequence learning models have been
analysed according to this framework. The network model is an associative
memory with a separate store for the sequence context of a symbol. A sparse
distributed memory is used to gain scalability. The context store combines the
functionality of a neural layer with a shift register. The sensitivity of the
machine to the sequence context is controllable, resulting in different
characteristic behaviours. The model can store and predict on-line sequences of
various types and length. Numerical simulations on the model have been carried
out to determine its properties.Comment: Published in IJCNN 2005, Montreal, Canad
Glaciological and volcanological studies in the Wrangell Mountains, Alaska
There are no author-identified significant results in this report
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