7,397 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
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
On the Weyl transverse frames in type I spacetimes
We apply a covariant and generic procedure to obtain explicit expressions of
the transverse frames that a type I spacetime admits in terms of an arbitrary
initial frame. We also present a simple and general algorithm to obtain the
Weyl scalars , and associated with these
transverse frames. In both cases it is only necessary to choose a particular
root of a cubic expression.Comment: 12 pages, submitted to Gen. Rel. Grav. (6-3-2004
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
Heavily obscured AGN with SIMBOL-X
By comparing an optically selected sample of narrow lines AGN with an X-ray
selected sample of AGN we have recently derived an estimate of the intrinsic
(i.e. before absorption) 2-10 keV luminosity function (XLF) of Compton Thick
AGNs. We will use this XLF to derive the number of Compton Thick AGN that will
be found in the SIMBOL-X survey(s).Comment: Talk at the Simbol-X symposium held in Paris, 2-5 December, 2008. 6
pages, 1 figure with three panel
Pair-correlation function in 2-dimensional lattice gases
The pair-correlation function in two-dimensional lattice gases is computed by means of three discretized classical equations for the structure of liquids: the hypernetted-chain, the Percus-Yevick, and the crossover integral equations. The equations are numerically solved by an iteration procedure. Two different systems are considered: the Ising-Peierls lattice gas with nearest-neighbor interactions and a model for O adsorbed on the W(110) surface, in which interactions up to the fourth neighbors are taken into account. The values of the pair-correlation function for nearest, next-nearest, and next-next-nearest neighbors are compared with the results of Monte Carlo simulations at four different coverages Θ (Θ=1/8, 1) / 4 ,1/2,3/4) as functions of the lateral coupling. It turns out that the crossover integral equation gives the best agreement with Monte Carlo data in both systems, being accurate especially at low Θ, whereas the Percus-Yevick equation fails in a wide range of parameters
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
On the invariant symmetries of the -metrics
We analyze the symmetries and other invariant qualities of the
-metrics (type D aligned Einstein Maxwell solutions with
cosmological constant whose Debever null principal directions determine
shear-free geodesic null congruences). We recover some properties and deduce
new ones about their isometry group and about their quadratic first integrals
of the geodesic equation, and we analyze when these invariant symmetries
characterize the family of metrics. We show that the subfamily of the Kerr-NUT
solutions are those admitting a Papapetrou field aligned with the Weyl tensor.Comment: 18 pages; v2: minor change
The emerging population of pulsar wind nebulae in hard X-rays
The hard X-ray synchrotron emission from pulsar wind nebulae (PWNe) probes
energetic particles, closely related to the pulsar injection power at the
present time. INTEGRAL has disclosed the yet poorly known population of hard
X-ray pulsar/PWN systems. We summarize the properties of the class, with
emphasys on the first hard X-ray bow-shock (CTB 80 powered by PSR B1951+32),
and highlight some prospects for the study of Pulsar Wind Nebulae with the
Simbol-X mission.Comment: Proceedings of the 2nd Simbol-X Symposium, AIP Conf. Proc. Series,
Eds. P. Ferrando and J. Rodriguez (4 pages, 2 figures
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