6,934 research outputs found
Obtaining the Weyl tensor from the Bel-Robinson tensor
The algebraic study of the Bel-Robinson tensor proposed and initiated in a
previous work (Gen. Relativ. Gravit. {\bf 41}, see ref [11]) is achieved. The
canonical form of the different algebraic types is obtained in terms of
Bel-Robinson eigen-tensors. An algorithmic determination of the Weyl tensor
from the Bel-Robinson tensor is presented.Comment: 21 page
Vacuum type I spacetimes and aligned Papapetrou fields: symmetries
We analyze type I vacuum solutions admitting an isometry whose Killing
2--form is aligned with a principal bivector of the Weyl tensor, and we show
that these solutions belong to a family of type I metrics which admit a group
of isometries. We give a classification of this family and we study the
Bianchi type for each class. The classes compatible with an aligned Killing
2--form are also determined. The Szekeres-Brans theorem is extended to non
vacuum spacetimes with vanishing Cotton tensor.Comment: 19 pages; a reference adde
An intrinsic characterization of 2+2 warped spacetimes
We give several equivalent conditions that characterize the 2+2 warped
spacetimes: imposing the existence of a Killing-Yano tensor subject to
complementary algebraic restrictions; in terms of the projector (or of the
canonical 2-form ) associated with the 2-planes of the warped product. These
planes are principal planes of the Weyl and/or Ricci tensors and can be
explicitly obtained from them. Therefore, we obtain the necessary and
sufficient (local) conditions for a metric tensor to be a 2+2 warped product.
These conditions exclusively involve explicit concomitants of the Riemann
tensor. We present a similar analysis for the conformally 2+2 product
spacetimes and give an invariant classification of them. The warped products
correspond to two of these invariant classes. The more degenerate class is the
set of product metrics which are also studied from an invariant point of view.Comment: 18 pages; submitted to Class. Quantum Grav
Rainich theory for type D aligned Einstein-Maxwell solutions
The original Rainich theory for the non-null Einstein-Maxwell solutions
consists of a set of algebraic conditions and the Rainich (differential)
equation. We show here that the subclass of type D aligned solutions can be
characterized just by algebraic restrictions.Comment: 12 pages; v2: appendix with notatio
Forward-backward equations for nonlinear propagation in axially-invariant optical systems
We present a novel general framework to deal with forward and backward
components of the electromagnetic field in axially-invariant nonlinear optical
systems, which include those having any type of linear or nonlinear transverse
inhomogeneities. With a minimum amount of approximations, we obtain a system of
two first-order equations for forward and backward components explicitly
showing the nonlinear couplings among them. The modal approach used allows for
an effective reduction of the dimensionality of the original problem from 3+1
(three spatial dimensions plus one time dimension) to 1+1 (one spatial
dimension plus one frequency dimension). The new equations can be written in a
spinor Dirac-like form, out of which conserved quantities can be calculated in
an elegant manner. Finally, these new equations inherently incorporate
spatio-temporal couplings, so that they can be easily particularized to deal
with purely temporal or purely spatial effects. Nonlinear forward pulse
propagation and non-paraxial evolution of spatial structures are analyzed as
examples.Comment: 11 page
On the classification of type D spacetimes
We give a classification of the type D spacetimes based on the invariant
differential properties of the Weyl principal structure. Our classification is
established using tensorial invariants of the Weyl tensor and, consequently,
besides its intrinsic nature, it is valid for the whole set of the type D
metrics and it applies on both, vacuum and non-vacuum solutions. We consider
the Cotton-zero type D metrics and we study the classes that are compatible
with this condition. The subfamily of spacetimes with constant argument of the
Weyl eigenvalue is analyzed in more detail by offering a canonical expression
for the metric tensor and by giving a generalization of some results about the
non-existence of purely magnetic solutions. The usefulness of these results is
illustrated in characterizing and classifying a family of Einstein-Maxwell
solutions. Our approach permits us to give intrinsic and explicit conditions
that label every metric, obtaining in this way an operational algorithm to
detect them. In particular a characterization of the Reissner-Nordstr\"{o}m
metric is accomplished.Comment: 29 pages, 0 figure
What can Simbol-X do for gamma-ray binaries?
Gamma-ray binaries have been uncovered as a new class of Galactic objects in
the very high energy sky (> 100 GeV). The three systems known today have hard
X-ray spectra (photon index ~ 1.5), extended radio emission and a high
luminosity in gamma-rays. Recent monitoring campaigns of LSI +61 303 in X-rays
have confirmed variability in these systems and revealed a spectral hardening
with increasing flux. In a generic one-zone leptonic model, the cooling of
relativistic electrons accounts for the main spectral and temporal features
observed at high energy. Persistent hard X-ray emission is expected to extend
well beyond 10 keV. We explain how Simbol-X will constrain the existing models
in connection with Fermi Space Telescope measurements. Because of its
unprecedented sensitivity in hard X-rays, Simbol-X will also play a role in the
discovery of new gamma-ray binaries, giving new insights into the evolution of
compact binaries.Comment: 4 pages, 1 figure, Proceedings of the 2nd International Simbol-X
symposium held in Paris, 2-5 December 200
Simbol-X Background Minimization: Mirror Spacecraft Passive Shielding Trade-Off Study
The present work shows a quantitative trade-off analysis of the Simbol-X
Mirror Spacecraft (MSC) passive shielding, in the phase space of the various
parameters: mass budget, dimension, geometry, and composition. A simplified
physical (and geometrical) model of the sky screen, implemented by means of a
GEANT4 simulation, has been developed to perform a performance-driven mass
optimization and evaluate the residual background level on Simbol-X focal
plane.Comment: 3 pages, 6 figures, to appear in the proceedings of the second
Simbol-X International Symposium "Simbol-X - Focusing on the Hard X-ray
Universe", AIP Conf. Proc. Series, P. Ferrando and J. Rodriguez ed
High Momentum Probes of Nuclear Matter
We discuss how the chemical composition of QCD jets is altered by final state
interactions in surrounding nuclear matter. We describe this process through
conversions of leading jet particles. We find that conversions lead to an
enhancement of kaons at high transverse momentum in Au+Au collisions at RHIC,
while their azimuthal asymmetry v_2 is suppressed.Comment: Contribution to the 4th international workshop High-pT physics at LHC
09, Prague; 6 pages, 6 figure
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