2,252 research outputs found
Algorithmic construction of static perfect fluid spheres
Perfect fluid spheres, both Newtonian and relativistic, have attracted
considerable attention as the first step in developing realistic stellar models
(or models for fluid planets). Whereas there have been some early hints on how
one might find general solutions to the perfect fluid constraint in the absence
of a specific equation of state, explicit and fully general solutions of the
perfect fluid constraint have only very recently been developed. In this
article we present a version of Lake's algorithm [Phys. Rev. D 67 (2003)
104015; gr-qc/0209104] wherein: (1) we re-cast the algorithm in terms of
variables with a clear physical meaning -- the average density and the locally
measured acceleration due to gravity, (2) we present explicit and fully general
formulae for the mass profile and pressure profile, and (3) we present an
explicit closed-form expression for the central pressure. Furthermore we can
then use the formalism to easily understand the pattern of inter-relationships
among many of the previously known exact solutions, and generate several new
exact solutions.Comment: Uses revtex4. V2: Minor clarifications, plus an additional section on
how to turn the algorithm into a solution generalization technique. This
version accepted for publication in Physical Review D. Now 7 page
Hydromagnetic and gravitomagnetic crust-core coupling in a precessing neutron star
We consider two types of mechanical coupling between the crust and the core
of a precessing neutron star. First, we find that a hydromagnetic (MHD)
coupling between the crust and the core strongly modifies the star's
precessional modes when ; here is the
Alfven crossing timescale, and and are the star's spin and
precession periods, respectively. We argue that in a precessing pulsar PSR
B1828-11 the restoring MHD stress prevents a free wobble of the crust relative
to the non-precessing core. Instead, the crust and the proton-electron plasma
in the core must precess in unison, and their combined ellipticity determines
the period of precession. Link has recently shown that the neutron superfluid
vortices in the core of PSR B1828-11 cannot be pinned to the plasma; he has
also argued that this lack of pinning is expected if the proton Fermi liquid in
the core is type-I superconductor. In this case, the neutron superfluid is
dynamically decoupled from the precessing motion. The pulsar's precession
decays due to the mutual friction between the neutron superfluid and the plasma
in the core. The decay is expected to occur over tens to hundreds of precession
periods and may be measurable over a human lifetime. Such a measurement would
provide information about the strong n-p interaction in the neutron-star core.
Second, we consider the effect of gravitomagnetic coupling between the neutron
superfluid in the core and the rest of the star and show that this coupling
changes the rate of precession by about 10%. The general formalism developed in
this paper may be useful for other applications.Comment: 6 page
Gravitational radiation from dynamical black holes
An effective energy tensor for gravitational radiation is identified for
uniformly expanding flows of the Hawking mass-energy. It appears in an energy
conservation law expressing the change in mass due to the energy densities of
matter and gravitational radiation, with respect to a Killing-like vector
encoding a preferred flow of time outside a black hole. In a spin-coefficient
formulation, the components of the effective energy tensor can be understood as
the energy densities of ingoing and outgoing, transverse and longitudinal
gravitational radiation. By anchoring the flow to the trapping horizon of a
black hole in a given sequence of spatial hypersurfaces, there is a locally
unique flow and a measure of gravitational radiation in the strong-field
regime.Comment: 5 revtex4 pages. Additional comment
A note on the peeling theorem in higher dimensions
We demonstrate the ``peeling property'' of the Weyl tensor in higher
dimensions in the case of even dimensions (and with some additional
assumptions), thereby providing a first step towards understanding of the
general peeling behaviour of the Weyl tensor, and the asymptotic structure at
null infinity, in higher dimensions.Comment: 5 pages, to appear in Class. Quantum Gra
Non-radial oscillations of anisotropic neutron stars in the Cowling approximation
One of the most common assumptions in the study of neutron star models and
their oscillations is that the pressure is isotopic, however there are
arguments that this may not be correct. Thus in the present paper we make a
first step towards studying the nonradial oscillations of neutron stars with an
anisotropic pressure. We adopt the so-called Cowling approximation where the
spacetime metric is kept fixed and the oscillation spectrum for the first few
fluid modes is obtained. The effect of the anisotropy on the frequencies is
apparent, although with the present results it might be hard to distinguish it
from the changes in the frequencies caused by different equations of state.Comment: 17 pages, 8 figures; title changed, comments adde
Chandra detection of the radio and optical double hot spot of 3C 351
In this letter we report a Chandra X-ray detection of the double northern hot
spot of the radio quasar 3C 351. The hot spot has also been observed in the
optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio
Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are
interpreted as the results of the synchrotron and synchrotron-self-Compton
(SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times
smaller than the equipartition values. In the framework of shock acceleration
theory, we show that the requirement for such a relatively small field strength
is in agreement with the fitted synchrotron spectral models and with the sizes
of the hot spots. Finally, we show that the combination of a lower magnetic
field strength with the high frequencies of the synchrotron cut-off in the
fitted synchrotron spectra provides strong evidence for electron acceleration
in the hot spots.Comment: 16 pag. + 2 .PS figures (fig.2 color), ApJ Letter in pres
On a choice of the Bondi radial coordinate and news function for the axisymmetric two-body problem
In the Bondi formulation of the axisymmetric vacuum Einstein equations, we
argue that the ``surface area'' coordinate condition determining the ``radial''
coordinate can be considered as part of the initial data and should be chosen
in a way that gives information about the physical problem whose solution is
sought. For the two-body problem, we choose this coordinate by imposing a
condition that allows it to be interpreted, near infinity, as the (inverse of
the) Newtonian potential. In this way, two quantities that specify the problem
-- the separation of the two particles and their mass ratio -- enter the
equations from the very beginning. The asymptotic solution (near infinity) is
obtained and a natural identification of the Bondi "news function" in terms of
the source parameters is suggested, leading to an expression for the radiated
energy that differs from the standard quadrupole formula but agrees with recent
non-linear calculations. When the free function of time describing the
separation of the two particles is chosen so as to make the new expression
agree with the classical result, closed-form analytic expressions are obtained,
the resulting metric approaching the Schwarzschild solution with time. As all
physical quantities are defined with respect to the flat metric at infinity,
the physical interpretation of this solution depends strongly on how these
definitions are extended to the near-zone and, in particular, how the "time"
function in the near-zone is related to Bondi's null coordinate.Comment: 13 pages, LaTeX, submitted to Classical and Quantum Gravity; v2
corrected a few typos and added some comments; v3 expanded discussion and
added references -- Rejected by CQG; v4: 8 pages revtex4 2 column,
extensively revised, submitted to Phys Rev D; v5: 21 pages revtex4 preprint;
further discussion of physical interpretation; v6: 21 pages revtex4 preprint
-- final version to appear in Phys. Rev. D (2006
A General Relativistic Magnetohydrodynamics Simulation of Jet Formation
We have performed a fully three-dimensional general relativistic
magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion
disk around a Schwarzschild black hole with a free-falling corona. The initial
simulation results show that a bipolar jet (velocity ) is created as
shown by previous two-dimensional axisymmetric simulations with mirror symmetry
at the equator. The 3-D simulation ran over one hundred light-crossing time
units ( where ) which is
considerably longer than the previous simulations. We show that the jet is
initially formed as predicted due in part to magnetic pressure from the
twisting the initially uniform magnetic field and from gas pressure associated
with shock formation in the region around . At later times,
the accretion disk becomes thick and the jet fades resulting in a wind that is
ejected from the surface of the thickened (torus-like) disk. It should be noted
that no streaming matter from a donor is included at the outer boundary in the
simulation (an isolated black hole not binary black hole). The wind flows
outwards with a wider angle than the initial jet. The widening of the jet is
consistent with the outward moving torsional Alfv\'{e}n waves (TAWs). This
evolution of disk-jet coupling suggests that the jet fades with a thickened
accretion disk due to the lack of streaming material from an accompanying star.Comment: 27 pages, 8 figures, revised and accepted to ApJ (figures with better
resolution: http://gammaray.nsstc.nasa.gov/~nishikawa/schb1.pdf
Energy-Momentum of a regular MMaS-class black hole
We compute the energy and momentum of a regular black hole of type defined by
Mars, Martin-Prats, and Senovilla using the Einstein and Papapetrou definitions
for energy-momentum density. Some other definitions of energy-momentum density
are shown to give mutually contradictory and less reasonable results. Results
support the Cooperstock hypothesis.Comment: 16 pages, 3 figures, LaTex2e; made minor corrections (in content and
in references) at the behest of two anonymous referees. Paper to appear in
IJMP
Perturbations on steady spherical accretion in Schwarzschild geometry
The stationary background flow in the spherically symmetric infall of a
compressible fluid, coupled to the space-time defined by the static
Schwarzschild metric, has been subjected to linearized perturbations. The
perturbative procedure is based on the continuity condition and it shows that
the coupling of the flow with the geometry of space-time brings about greater
stability for the flow, to the extent that the amplitude of the perturbation,
treated as a standing wave, decays in time, as opposed to the amplitude
remaining constant in the Newtonian limit. In qualitative terms this situation
simulates the effect of a dissipative mechanism in the classical Bondi
accretion flow, defined in the Newtonian construct of space and time. As a
result of this approach it becomes impossible to define an acoustic metric for
a conserved spherically symmetric flow, described within the framework of
Schwarzschild geometry. In keeping with this view, the perturbation, considered
separately as a high-frequency travelling wave, also has its amplitude reduced.Comment: 8 pages, no figur
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