21,700 research outputs found
Measuring multipole moments of Weyl metrics by means of gyroscopes
Using the technique of Rindler and Perlick we calculate the total precession
per revolution of a gyroscope circumventing the source of Weyl metrics. We
establish thereby a link between the multipole moments of the source and an
``observable'' quantity. Special attention deserves the case of the
gamma-metric. As an extension of this result we also present the corresponding
expressions for some stationary space-times.Comment: 18 pages Latex, To appear in J.Math.Phy
Gravitational fields of axially symmetric compact objects in 5D space-time-matter gravity
In the standard Einstein's theory the exterior gravitational field of any
static and axially symmetric stellar object can be described by means of a
single function from which we obtain a metric into a four-dimensional
space-time. In this work we present a generalization of those so called Weyl
solutions to a space-time-matter metric in a five-dimensional manifold within a
non-compactified Kaluza-Klein theory of gravity. The arising field equations
reduce to those of vacuum Einstein's gravity when the metric function
associated to the fifth dimension is considered to be constant. The calculation
of the geodesics allows to identify the existence or not of different
behaviours of test particles, in orbits on a constant plane, between the two
metrics. In addition, static solutions on the hypersurface orthogonal to the
added dimension but with time dependence in the five-dimensional metric are
also obtained. The consequences on the variation of the rest mass, if the fifth
dimension is identified with it, are studied.Comment: 20 pages. No figures. General Relativity and Gravitation (2024) 56:2
Quadratic Maps in Two Variables on Arbitrary Fields
Let be a field of characteristic different from and , and
let be a vector space of dimension over . The generic
classification of homogeneous quadratic maps under the action
of the linear group of , is given and efficient computational criteria to
recognize equivalence are provided.Comment: 12 pages, no figure
Watson's theorem and the axial transition
We present a new determination of the axial form factors from
neutrino induced pion production data. For this purpose, the model of Hernandez
{\it et al.} [Phys. Rev. D76, 033005 (2007)] is improved by partially restoring
unitarity. This is accomplished by imposing Watson's theorem on the dominant
vector and axial multipoles. As a consequence, a larger , in good
agreement with the prediction from the off-diagonal Goldberger-Treiman
relation, is now obtained.Comment: 17 latex pages, 7 figures, 2 tables. Some typos correcte
Approximate gravitational field of a rotating deformed mass
A new approximate solution of vacuum and stationary Einstein field equations
is obtained. This solution is constructed by means of a power series expansion
of the Ernst potential in terms of two independent and dimensionless parameters
representing the quadrupole and the angular momentum respectively. The main
feature of the solution is a suitable description of small deviations from
spherical symmetry through perturbations of the static configuration and the
massive multipole structure by using those parameters. This quality of the
solution might eventually provide relevant differences with respect to the
description provided by the Kerr solution.Comment: 16 pages. Latex. To appear in General Relativity and Gravitatio
X-ray spectral variability of seven LINER nuclei with XMM-Newton and Chandra data
One of the most important features in active galactic nuclei (AGN) is the
variability of their emission. Variability has been discovered at X-ray, UV,
and radio frequencies on time scales from hours to years. Among the AGN family
and according to theoretical studies, Low-Ionization Nuclear Emission Line
Region (LINER) nuclei would be variable objects on long time scales. Our
purpose is to investigate spectral X-ray variability in LINERs and to
understand the nature of these kinds of objects, as well as their accretion
mechanism. Chandra and XMM-Newton public archives were used to compile X-ray
spectra of seven LINER nuclei at different epochs with time scales of years. To
search for variability we fit all the spectra from the same object with a set
of models, in order to identify the parameters responsible for the variability
pattern. We also analyzed the light curves in order to search for short time
scale (from hours to days) variability. Whenever possible, UV variability was
also studied. We found spectral variability in four objects, with variations
mostly related to hard energies (2-10 keV). These variations are due to changes
in the soft excess, and/or changes in the absorber, and/or intrinsic variations
of the source. Another two galaxies seem not to vary. Short time scale
variations during individual observations were not found. Our analysis confirms
the previously reported anticorrelation between the X-ray spectral index and
the Eddington ratio, and also the correlation between the X-ray to UV flux
ratio and the Eddington ratio. These results support an Advection Dominated
Accretion Flow (ADAF) as the accretion mechanism in LINERs.Comment: 35 pages, 53 figures, recently accepted pape
X-ray spectral variability of Seyfert 2 galaxies
Variability across the electromagnetic spectrum is a property of AGN that can
help constraining the physical properties of these galaxies. This is the third
of a serie of papers with the aim of studying the X-ray variability of
different families of AGN. The main purpose of this work is to investigate the
variability pattern in a sample of optically selected type 2 Seyfert galaxies.
We use the 26 Seyferts in the Veron-Cetty and Veron catalogue with data
available from Chandra and/or XMM-Newton public archives at different epochs,
with timescales ranging from a few hours to years. All the spectra of the same
source are simultaneously fitted and we let different parameters to vary in the
model. Whenever possible, short-term variations and/or long-term UV flux
variations are studied. We divide the sample in Compton-thick, Compton-thin,
and changing-look candidates. Short-term variability at X-rays is not found.
From the 25 analyzed sources, 11 show long-term variations; eight (out of 11)
are Compton-thin, one (out of 12) is Compton-thick, and the two changing-look
candidates are also variable. The main driver for the X-ray changes is related
to the nuclear power (nine cases), while variations at soft energies or related
with absorbers at hard X-rays are less common, and in many cases these
variations are accompained with variations of the nuclear continuum. At UV
frequencies nuclear variations are nor found. We report for the first time two
changing-look candidates, MARK273 and NGC7319. A constant reflection component
located far away from the nucleus plus a variable nuclear continuum are able to
explain most of our results; the Compton-thick candidates are dominated by
reflection, which supresses their continuum making them seem fainter, and not
showing variations, while the Compton-thin and changing-look candidates show
variations.Comment: Accepted for publication in A&
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