4,728 research outputs found
Collisions of rigidly rotating disks of dust in General Relativity
We discuss inelastic collisions of two rotating disks by using the
conservation laws for baryonic mass and angular momentum. In particular, we
formulate conditions for the formation of a new disk after the collision and
calculate the total energy loss to obtain upper limits for the emitted
gravitational energy.Comment: 30 pages, 9 figure
Non-existence of stationary two-black-hole configurations: The degenerate case
In a preceding paper we examined the question whether the spin-spin repulsion
and the gravitational attraction of two aligned sub-extremal black holes can
balance each other. Based on the solution of a boundary value problem for two
separate (Killing-) horizons and a novel black hole criterion we were able to
prove the non-existence of the equilibrium configuration in question. In this
paper we extend the non-existence proof to extremal black holes.Comment: 18 pages, 2 figure
Spin-Orbit Coupling and the Evolution of Transverse Spin
We investigate the evolution of transverse spin in tightly focused circularly
polarized beams of light, where spin-orbit coupling causes a local rotation of
the polarization ellipses upon propagation through the focal volume. The effect
can be explained as a relative Gouy-phase shift between the circularly
polarized transverse field and the longitudinal field carrying orbital angular
momentum. The corresponding rotation of the electric transverse spin density is
observed experimentally by utilizing a recently developed reconstruction
scheme, which relies on transverse-spin-dependent directional scattering of a
nano-probe.Comment: 4 pages, 2 figure
The interior of axisymmetric and stationary black holes: Numerical and analytical studies
We investigate the interior hyperbolic region of axisymmetric and stationary
black holes surrounded by a matter distribution. First, we treat the
corresponding initial value problem of the hyperbolic Einstein equations
numerically in terms of a single-domain fully pseudo-spectral scheme.
Thereafter, a rigorous mathematical approach is given, in which soliton methods
are utilized to derive an explicit relation between the event horizon and an
inner Cauchy horizon. This horizon arises as the boundary of the future domain
of dependence of the event horizon. Our numerical studies provide strong
evidence for the validity of the universal relation \Ap\Am = (8\pi J)^2 where
\Ap and \Am are the areas of event and inner Cauchy horizon respectively,
and denotes the angular momentum. With our analytical considerations we are
able to prove this relation rigorously.Comment: Proceedings of the Spanish Relativity Meeting ERE 2010, 10 pages, 5
figure
Analytical approximation of the exterior gravitational field of rotating neutron stars
It is known that B\"acklund transformations can be used to generate
stationary axisymmetric solutions of Einstein's vacuum field equations with any
number of constants. We will use this class of exact solutions to describe the
exterior vacuum region of numerically calculated neutron stars. Therefore we
study how an Ernst potential given on the rotation axis and containing an
arbitrary number of constants can be used to determine the metric everywhere.
Then we review two methods to determine those constants from a numerically
calculated solution. Finally, we compare the metric and physical properties of
our analytic solution with the numerical data and find excellent agreement even
for a small number of parameters.Comment: 9 pages, 10 figures, 3 table
Stability of general relativistic Miyamoto-Nagai galaxies
The stability of a recently proposed general relativistic model of galaxies
is studied in some detail. This model is a general relativistic version of the
well known Miyamoto-Nagai model that represents well a thick galactic disk. The
stability of the disk is investigated under a general first order perturbation
keeping the spacetime metric frozen (no gravitational radiation is taken into
account). We find that the stability is associated with the thickness of the
disk. We have that flat galaxies have more not-stable modes than the thick ones
i.e., flat galaxies have a tendency to form more complex structures like rings,
bars and spiral arms.Comment: 11 pages, 5 figures, accepted for publication in MNRA
Relativistic Models of Galaxies
A special form of the isotropic metric in cylindrical coordinates is used to
construct what may be interpreted as the General Relativistic versions of some
wellknown potential-density pairs used in Newtonian gravity to model
three-dimensional distributions of matter in galaxies. The components of the
energy-momentum tensor are calculated for the first two Miyamoto-Nagai
potentials and a particular potential due to Satoh. The three potentials yield
distributions of matter in which all tensions are pressures and all energy
conditions are satisfied for certain ranges of the free parameters. A few
non-planar geodesic orbits are computed for one of the potentials and compared
with the Newtonian case. Rotation is also incorporated to the models and the
effects of the source rotation on the rotation profile are calculated as first
order corrections by using an approximate form of the Kerr metric in isotropic
coordinates.Comment: 18 pages, 23 eps figures, uses mn2e.cls style file, to be published
in MNRA
Clean and As-covered zinc-blende GaN (001) surfaces: Novel surface structures and surfactant behavior
We have investigated clean and As-covered zinc-blende GaN (001) surfaces,
employing first-principles total-energy calculations. For clean GaN surfaces
our results reveal a novel surface structure very different from the
well-established dimer structures commonly observed on polar III-V (001)
surfaces: The energetically most stable surface is achieved by a Peierls
distortion of the truncated (1x1) surface rather than through addition or
removal of atoms. This surface exhibits a (1x4) reconstruction consisting of
linear Ga tetramers. Furthermore, we find that a submonolayer of arsenic
significantly lowers the surface energy indicating that As may be a good
surfactant. Analyzing surface energies and band structures we identify the
mechanisms which govern these unusual structures and discuss how they might
affect growth properties.Comment: 4 pages, 3 figures, to be published in Appears in Phys. Rev. Lett.
(in print). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
A Luminous Companion to SGR 1806-20
We have obtained infrared spectra of the star suggested to be the counterpart
of the soft gamma-ray repeater (SGR) 1806-20. We found strong emission lines
similar to those seen in the spectra of the rare Luminous Blue Variables and
B[e] stars. A He I absorption line is also seen, from which we infer a spectral
type O9--B2. This classification, in combination with the minimum distance of
\simgt6 kpc inferred from its extinction, makes the star one of the most
luminous in the Galaxy. We infer that it is a companion to SGR 1806-20, and
suggest that the presence of a companion is somehow related to the SGR
phenomenon.Comment: 5 pages, AASTEX text+table and 2 PostScript figures (needs LaTeX
style files aaspptwo.sty, epsf.sty and rotate.sty). In case of problems,
contact [email protected]. Postscript file of complete article available
on request. (Replaced because first version had one wrong reference in it
- …