3,200 research outputs found
An Iterative Approach to Twisting and Diverging, Type N, Vacuum Einstein Equations: A (Third-Order) Resolution of Stephani's `Paradox'
In 1993, a proof was published, within ``Classical and Quantum Gravity,''
that there are no regular solutions to the {\it linearized} version of the
twisting, type-N, vacuum solutions of the Einstein field equations. While this
proof is certainly correct, we show that the conclusions drawn from that fact
were unwarranted, namely that this irregularity caused such solutions not to be
able to truly describe pure gravitational waves. In this article, we resolve
the paradox---since such first-order solutions must always have singular lines
in space for all sufficiently large values of ---by showing that if we
perturbatively iterate the solution up to the third order in small quantities,
there are acceptable regular solutions. That these solutions become flat before
they become non-twisting tells us something interesting concerning the general
behavior of solutions describing gravitational radiation from a bounded source.Comment: 11 pages, a plain TeX file, submitted to ``Classical and Quantum
Gravity'
A comparison of DA white dwarf temperatures and gravities from Lyman and Balmer line studies
We present measurements of the effective temperatures and surface gravities
for a sample of hot DA white dwarfs, using the Lyman line data available from
the HUT, ORFEUS and FUSE far-UV space missions. Comparing the results with
those from the standard Balmer line technique, we find that there is a general
good overall agreement between the two methods. However, significant
differences are found for a number of stars, but not always of a consistent
nature in that sometimes the Balmer temperature exceeds that derived from the
Lyman lines and in other instances is lower. We conclude that, with the latest
model atmosphere calculations, these discrepancies probably do not arise from
an inadequate theoretical treatment of the Lyman lines but rather from
systematic effects in the observation and data reduction processes, which
dominate the statistical errors in these spectra. If these systematic data
reduction effects can be adequately controlled, the Lyman line temperature and
gravity measurements are consistent with those obtained from the Balmer lines
when allowance is made for reasonable observational uncertainties.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society 14 pages, 13 figure
New first integral for twisting type-N vacuum gravitational fields with two non-commuting Killing vectors
A new first integral for the equations corresponding to twisting type-N
vacuum gravitational fields with two non-commuting Killing vectors is
introduced. A new reduction of the problem to a complex second-order ordinary
differential equation is given. Alternatively, the mentioned first integral can
be used in order to provide a first integral of the second-order complex
equation introduced in a previous treatment of the problem.Comment: 7 pages, LaTeX, uses ioplppt.sty and iopl12.sty; to be published in
Class. Quantum Gra
Using the local density approximation and the LYP, BLYP, and B3LYP functionals within Reference--State One--Particle Density--Matrix Theory
For closed-shell systems, the local density approximation (LDA) and the LYP,
BLYP, and B3LYP functionals are shown to be compatible with reference-state
one-particle density-matrix theory, where this recently introduced formalism is
based on Brueckner-orbital theory and an energy functional that includes exact
exchange and a non-universal correlation-energy functional. The method is
demonstrated to reduce to a density functional theory when the
exchange-correlation energy-functional has a simplified form, i.e., its
integrand contains only the coordinates of two electron, say r1 and r2, and it
has a Dirac delta function -- delta(r1 - r2) -- as a factor. Since Brueckner
and Hartree--Fock orbitals are often very similar, any local exchange
functional that works well with Hartree--Fock theory is a reasonable
approximation with reference-state one-particle density-matrix theory. The LDA
approximation is also a reasonable approximation. However, the Colle--Salvetti
correlation-energy functional, and the LYP variant, are not ideal for the
method, since these are universal functionals. Nevertheless, they appear to
provide reasonable approximations. The B3LYP functional is derived using a
linear combination of two functionals: One is the BLYP functional; the other
uses exact exchange and a correlation-energy functional from the LDA.Comment: 26 Pages, 0 figures, RevTeX 4, Submitted to Mol. Phy
Scalar second order evolution equations possessing an irreducible sl-valued zero curvature representation
We find all scalar second order evolution equations possessing an
sl-valued zero curvature representation that is not reducible to a proper
subalgebra of sl. None of these zero-curvature representations admits a
parameter.Comment: 10 pages, requires nath.st
The dispersive self-dual Einstein equations and the Toda lattice
The Boyer-Finley equation, or -Toda equation is both a reduction
of the self-dual Einstein equations and the dispersionlesslimit of the
-Toda lattice equation. This suggests that there should be a dispersive
version of the self-dual Einstein equation which both contains the Toda lattice
equation and whose dispersionless limit is the familiar self-dual Einstein
equation. Such a system is studied in this paper. The results are achieved by
using a deformation, based on an associative -product, of the algebra
used in the study of the undeformed, or dispersionless,
equations.Comment: 11 pages, LaTeX. To appear: J. Phys.
Joint searches between gravitational-wave interferometers and high-energy neutrino telescopes: science reach and analysis strategies
Many of the astrophysical sources and violent phenomena observed in our
Universe are potential emitters of gravitational waves (GWs) and high-energy
neutrinos (HENs). A network of GW detectors such as LIGO and Virgo can
determine the direction/time of GW bursts while the IceCube and ANTARES
neutrino telescopes can also provide accurate directional information for HEN
events. Requiring the consistency between both, totally independent, detection
channels shall enable new searches for cosmic events arriving from potential
common sources, of which many extra-galactic objects.Comment: 4 pages. To appear in the Proceedings of the 2d Heidelberg Workshop:
"High-Energy Gamma-rays and Neutrinos from Extra-Galactic Sources",
Heidelberg (Germany), January 13-16, 200
ASCA Detection of Pulsed X-ray Emission from PSR J0631+1036
ASCA's long look at the 288 millisecond radio pulsar, PSR J0631+1036, reveals
coherent X-ray pulsation from this source for the first time. The source was
first detected in the serendipitous Einstein observation and later identified
as a radio pulsar. Possible pulsation in the gamma-ray band has been detected
from the CGRO EGRET data (Zepka, et al. 1996). The X-ray spectrum in the ASCA
band is characterized by a hard power-law type emission with a photon index of
about 2.3, when fitted with a single power-law function modified with
absorption. An additional blackbody component of about 0.14 keV increases the
quality of the spectral fit. The observed X-ray flux is 2.1e-13 ergs/s/cm2 in
the 1-10 keV band. We find that many characteristics of PSR J0631+1036 are
similar to those of middle-aged gamma-ray pulsars such as PSR B1055-52, PSR
B0633+17 (Geminga), and PSR B0656+14.Comment: To appear in ApJ Letter
INTEGRAL long-term monitoring of the Supergiant Fast X-ray Transient XTE J1739-302
In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has
been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are
X-ray binary systems with a compact companion orbiting a supergiant star which
show very short and bright outbursts in a series of activity periods
overimposed on longer quiescent periods. Only very recently the first attempts
to model the behaviour of these sources have been published, some of them
within the framework of accretion from clumpy stellar winds.Our goal is to
analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of
the clumpy structure of the supergiant wind. We have used INTEGRAL and RXTE/PCA
observations in order to obtain broad band (1-200 keV) spectra and light curves
of XTE J1739-302 and investigate its X-ray spectrum and temporal variability.
We have found that XTE J1739-302 follows a much more complex behaviour than
expected. Far from presenting a regular variability pattern, XTE J1739-302
shows periods of high, intermediate, and low flaring activity.Comment: 9 pages, 7 figures, accepted for publication in A&
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