2,702 research outputs found
Sub-milliarcsec-scale structure of the gravitational lens B1600+434
In the gravitational lens system B1600+434 the brighter image, A, is known to
show rapid variability which is not detected in the weaker image, B (Koopmans &
de Bruyn 2000). Since correlated variability is one of the fundamental
properties of gravitational lensing, it has been proposed that image A is
microlensed by stars in the halo of the lensing galaxy (Koopmans & de Bruyn
2000). We present VLBA observations of B1600+434 at 15 GHz with a resolution of
0.5 milliarcsec to determine the source structure at high spatial resolution.
The surface brightness of the images are significantly different, with image A
being more compact. This is in apparent contradiction with the required
property of gravitational lensing that surface brightness be preserved. Our
results suggest that both the lensed images may show two-sided elongation at
this resolution, a morphology which does not necessarily favour superluminal
motion. Instead these data may suggest that image B is scatter-broadened at the
lens so that its size is larger than that of A, and hence scintillates less
than image A.Comment: 4 pages, 2 figures, accepted in AA Letter
Singularities in optimal structural design
Singularity conditions that arise during structural optimization can seriously degrade the performance of the optimizer. The singularities are intrinsic to the formulation of the structural optimization problem and are not associated with the method of analysis. Certain conditions that give rise to singularities have been identified in earlier papers, encompassing the entire structure. Further examination revealed more complex sets of conditions in which singularities occur. Some of these singularities are local in nature, being associated with only a segment of the structure. Moreover, the likelihood that one of these local singularities may arise during an optimization procedure can be much greater than that of the global singularity identified earlier. Examples are provided of these additional forms of singularities. A framework is also given in which these singularities can be recognized. In particular, the singularities can be identified by examination of the stress displacement relations along with the compatibility conditions and/or the displacement stress relations derived in the integrated force method of structural analysis
Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg
We present a catalog of 321 compact radio sources in the declination range
0deg > delta > -30deg. The positions of these sources have been measured with a
two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large
Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for
this catalog to be used mainly for selection of phase calibration sources for
radio interferometers, although compact radio sources have other scientific
uses.Comment: 9 pages. To appear in ApJS. Catalog (Table 3) is abbreviated in
printed version. Complete catalog available at
ftp://ftp.aoc.nrao.edu/pub/staff/jwrobel/WPW2003_ApJS.tx
Gigahertz Peaked Spectrum sources from the Jodrell Bank-VLA Astrometric Survey
Observations with MERLIN at 408 MHz have been used to establish the
low-frequency part of the spectra of more than a hundred compact radio sources
taken from the part of the Jodrell Bank--VLA Astrometric Survey limited by 35
deg. < \delta < 75 deg. These sources were selected from JVAS and other
catalogues to have convex spectra between 1.4 and 8.4 GHz, characteristic of
Gigahertz Peaked Spectrum (GPS) sources. We have confirmed convex shapes of the
spectra of 76 objects (one half of our initial candidates) thereby yielding the
largest genuine sample of GPS sources compiled so far. Seven of 17 identified
quasars in the sample have large (z\ga 2) redshifts.Comment: 18 pages, 2 figures, minor corrections. To appear in: A&A Suppl. vol.
135 (March 1999
Superconductivity at 11.3 K induced by cobalt doping in CeOFeAs
Pure phases of a new oxyarsenide superconductor of the nominal composition
CeOFe0.9Co0.1As was successfully synthesized by solid state reaction in sealed
silica ampoules at 1180 C. It crystallizes in the layered tetragonal ZrCuSiAs
type structure (sp gp P4/nmm) with lattice parameter of a = 3.9918(5) angstrom
and c = 8.603(1) angstrom. A sharp superconducting transition is observed at
11.31 K with an upper critical field of 45.22 T at ambient pressure. The
superconducting transition temperature is drastically lowered (~ 4.5, 4.9 K) on
increasing the concentration (x = 0.15, 0.2) of cobalt
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