2,345 research outputs found
Search for positron annihilation line and continuum radiation from the Galactic Center
Our balloon-borne germain gamma-ray telescope was flown over Alice Springs, Australia, on 1984 November 20 to search for the 511 keV positron annihilation line from the Galactic Center. The measured line flux at Earth was (0.6 + or - 4.4) x 0.001 ph/sq cm/s indicating that the source was still in a low or off state
Friedmann limits of rotating hypersurface-homogeneous dust models
The existence of Friedmann limits is systematically investigated for all the
hypersurface-homogeneous rotating dust models, presented in previous papers by
this author. Limiting transitions that involve a change of the Bianchi type are
included. Except for stationary models that obviously do not allow it, the
Friedmann limit expected for a given Bianchi type exists in all cases. Each of
the 3 Friedmann models has parents in the rotating class; the k = +1 model has
just one parent class, the other two each have several parent classes. The type
IX class is the one investigated in 1951 by Goedel. For each model, the
consecutive limits of zero rotation, zero tilt, zero shear and spatial isotropy
are explicitly calculated.Comment: 39 pages, LaTeX, 1 postscript figure. Subjects: General relativity,
exact solutions, cosmolog
Properties of kinematic singularities
The locally rotationally symmetric tilted perfect fluid Bianchi type V
cosmological model provides examples of future geodesically complete spacetimes
that admit a `kinematic singularity' at which the fluid congruence is
inextendible but all frame components of the Weyl and Ricci tensors remain
bounded. We show that for any positive integer n there are examples of Bianchi
type V spacetimes admitting a kinematic singularity such that the covariant
derivatives of the Weyl and Ricci tensors up to the n-th order also stay
bounded. We briefly discuss singularities in classical spacetimes.Comment: 13 pages. Published version. One sentence from version 2 correcte
Cosmology, cohomology, and compactification
Ashtekar and Samuel have shown that Bianchi cosmological models with compact
spatial sections must be of Bianchi class A. Motivated by general results on
the symmetry reduction of variational principles, we show how to extend the
Ashtekar-Samuel results to the setting of weakly locally homogeneous spaces as
defined, e.g., by Singer and Thurston. In particular, it is shown that any
m-dimensional homogeneous space G/K admitting a G-invariant volume form will
allow a compact discrete quotient only if the Lie algebra cohomology of G
relative to K is non-vanishing at degree m.Comment: 6 pages, LaTe
Early-expressed chemokines predict kidney immunopathology in experimental disseminated Candida albicans infections
Available under the Creative Commons Attribution License (CCAL)Peer reviewedPublisher PD
Spectroscopy from 2 to 200 keV
The astrophysical processes responsible for line and continuum emission in the spectra range 2 keV to 200 keV are examined from the viewpoint of designing a spectrometer which would operate in this regime. Phenomena considered include fluorescent line radiation in X-ray binaries, magnetically shifted iron lines and cyclotron emission from neutron star surfaces, line emission from cosmically abundant elements in thermal plasmas, and nuclear deexcitation lines in fresh nucleosynthetically produced matter. An instrument consisting of a approximately 10 sq cm array of planar germanium detectors surrounded by a large sodium-iodide anticoincidence shield is described and projected background rates and sensitivities are considered. A sample observing program for a two-day shuttle-based mission is included as an example of the wide range of scientific questions which could be addressed by such an instrument
GRIS observations of the galactic center and the gamma ray galactic diffuse continuum
On two flights in 1988, the Gamma-Ray Imaging Spectrometer (GRIS) discovered the galactic center in a high state (greater than 1 x 10(exp -3) ph/(sq cm sec)) of positron annihilation line emission (511 keV) after nearly a decade of failed attempts to confirm the exciting early results of balloon and satellite instruments. These two flights represented the first flights of a new generation of high resolution germanium spectrometers designed to achieve significantly greater sensitivity for astrophysical observations. During the fall flight, an observation of the galactic plane at 335 degrees longitude was also performed. This observation showed a very low level of 511 keV emission (2 +/- 1 x 10(exp -4) ph/(sq cm sec)), confirming the galactic center origin of the line, and a high level of hard x-ray and gamma-ray continuum emission (1 x 10(exp -4) ph/(sq cm sec keV) at 100 keV), which we attribute to galactic diffuse emission. Improved fits to the spectrum of the galactic center are presented with the proposed diffuse component subtracted. We conclude that our galactic center continuum spectrum is consistent with the sum of the 1E1740.7-2942 spectrum observed by SIGMA/GRANAT and our 1 = 335 degree galactic plane spectrum. The predicted diffuse flux should be easily measurable by the Oriented Scintillation Spectrometer Experiment (OSSE) experiment on the Gamma Ray Observatory (GRO)
On the limits of Brans-Dicke spacetimes: a coordinate-free approach
We investigate the limit of Brans-Dicke spacetimes as the scalar field
coupling constant omega tends to infinity applying a coordinate-free technique.
We obtain the limits of some known exact solutions. It is shown that these
limits may not correspond to similar solutions in the general relativity
theory.Comment: LaTeX, 16 pp, report DF/UFPB/02-9
Equivalence of three-dimensional spacetimes
A solution to the equivalence problem in three-dimensional gravity is given
and a practically useful method to obtain a coordinate invariant description of
local geometry is presented. The method is a nontrivial adaptation of Karlhede
invariant classification of spacetimes of general relativity. The local
geometry is completely determined by the curvature tensor and a finite number
of its covariant derivatives in a frame where the components of the metric are
constants. The results are presented in the framework of real two-component
spinors in three-dimensional spacetimes, where the algebraic classifications of
the Ricci and Cotton-York spinors are given and their isotropy groups and
canonical forms are determined. As an application we discuss Goedel-type
spacetimes in three-dimensional General Relativity. The conditions for local
space and time homogeneity are derived and the equivalence of three-dimensional
Goedel-type spacetimes is studied and the results are compared with previous
works on four-dimensional Goedel-type spacetimes.Comment: 13 pages - content changes and corrected typo
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