19,874 research outputs found
ARCS, The Arcminute Radio Cluster-lens Search - I. Selection Criteria and Initial Results
We present the results of an unbiased radio search for gravitational lensing
events with image separations between 15 and 60 arcsec, which would be
associated with clusters of galaxies with masses >10^{13-14}M_{\sun}. A parent
population of 1023 extended radio sources stronger than 35 mJy with stellar
optical identifications was selected using the FIRST radio catalogue at 1.4 GHz
and the APM optical catalogue. The FIRST catalogue was then searched for
companions to the parent sources stronger than 7 mJy and with separation in the
range 15 to 60 arcsec. Higher resolution observations of the resulting 38 lens
candidates were made with the VLA at 1.4 GHz and 5 GHz, and with MERLIN at 5
GHz in order to test the lens hypothesis in each case. None of our targets was
found to be a gravitational lens system. These results provide the best current
constraint on the lensing rate for this angular scale, but improved
calculations of lensing rates from realistic simulations of the clustering of
matter on the relevant scales are required before cosmologically significant
constraints can be derived from this null result. We now have an efficient,
tested observational strategy with which it will be possible to make an
order-of-magnitude larger unbiased search in the near future.Comment: Accepted for publication in MNRAS. 12 pages, 29 included PostScript
figure
Synchrotron Polarization at High Galactic Latitude
We present preliminary results from mapping the high-latitude Galactic
polarization with the Effelsberg Telescope at 21 cm. Structures on the
resulting maps are mostly on the scale of several degrees. The results show
detection of polarization over most of the field, at the level of tens of
percent of the synchrotron emission. The evidence of more structure in Stokes Q
and U rather than in suggests the existence of Faraday
rotation.Comment: To be published in the proceedings of "The Cosmic Microwave
Background and its Polarization", New Astronomy Reviews, (eds. S. Hanany and
K.A. Olive
NICMOS and VLBA observations of the gravitational lens system B1933+503
NICMOS observations of the complex gravitational lens system B1933+503 reveal
infrared counterparts to two of the inverted spectrum radio images. The
infrared images have arc-like structures. The corresponding radio images are
also detected in a VLBA map made at 1.7 GHz with a resolution of 6 mas. We fail
to detect two of the four inverted radio spectrum components with the VLBA even
though they are clearly visible in a MERLIN map at the same frequency at a
different epoch. The absence of these two components could be due to rapid
variability on a time-scale less than the time delay, or to broadening of the
images during propagation of the radio waves through the ISM of the lensing
galaxy to an extent that they fall below the surface brightness detectability
threshold of the VLBA observations. The failure to detect the same two images
with NICMOS is probably due to extinction in the ISM of the lensing galaxy.Comment: 5 pages, 4 figures, submitted to MNRA
Evaluation of nonmetallic thermal protection materials for the manned space shuttle. Volume 1, task 1: Assessment of technical risks associated with utilization of nonmetallic thermal protection system
Technical problems of design and flight qualification of the proposed classes of surface insulation materials and leading edge materials were reviewed. A screening test plan, a preliminary design data test plan and a design data test plan were outlined. This program defined the apparent critical differences between the surface insulators and the leading edge materials, structuring specialized screening test plans for each of these two classes of materials. Unique testing techniques were shown to be important in evaluating the structural interaction aspects of the surface insulators and a separate task was defined to validate the test plan. In addition, a compilation was made of available information on proposed material (including metallic TPS), previous shuttle programs, pertinent test procedures, and other national programs of merit. This material was collected and summarized in an informally structured workbook
A survey of polarization in the JVAS/CLASS flat-spectrum radio source surveys: I. The data and catalogue production
We have used the very large JVAS/CLASS 8.4-GHz surveys of flat-spectrum radio
sources to obtain a large, uniformly observed and calibrated, sample of radio
source polarizations. These are useful for many investigations of the
properties of radio sources and the interstellar medium. We discuss comparisons
with polarization measurements from this survey and from other large-scale
surveys of polarization in flat-spectrum sources.Comment: Accepted by MNRAS. 8 pages, 5 figures. Full version of Table 2
available at http://www.jb.man.ac.uk/~njj/classqu_po
Gravitational lensing statistics with extragalactic surveys. II. Analysis of the Jodrell Bank-VLA Astrometric Survey
We present constraints on the cosmological constant from
gravitational lensing statistics of the Jodrell Bank-VLA Astrometric Survey
(JVAS). Although this is the largest gravitational lens survey which has been
analysed, cosmological constraints are only comparable to those from optical
surveys. This is due to the fact that the median source redshifts of JVAS are
lower, which leads to both relatively fewer lenses in the survey and a weaker
dependence on the cosmological parameters. Although more approximations have to
be made than is the case for optical surveys, the consistency of the results
with those from optical gravitational lens surveys and other cosmological tests
indicate that this is not a major source of uncertainty in the results.
However, joint constraints from a combination of radio and optical data are
much tighter. Thus, a similar analysis of the much larger Cosmic Lens All-Sky
Survey should provide even tighter constraints on the cosmological constant,
especially when combined with data from optical lens surveys.
At 95% confidence, our lower and upper limits on ,
using the JVAS lensing statistics information alone, are respectively -2.69 and
0.68. For a flat universe, these correspond to lower and upper limits on
\lambda_{0} of respectively -0.85 and 0.84. Using the combination of JVAS
lensing statistics and lensing statistics from the literature as discussed in
Quast & Helbig (Paper I) the corresponding values are
-1.78 and 0.27. For a flat universe, these correspond to lower and upper limits
on of respectively -0.39 and 0.64.Comment: LaTeX, 9 pages, 18 PostScript files in 6 figures. Paper version
available on request. Data available from
http://gladia.astro.rug.nl:8000/ceres/data_from_papers/papers.htm
Lattice Model of Sweeping Interface for Drying Process in Water-Granule Mixture
Based on the invasion percolation model, a lattice model for the sweeping
interface dynamics is constructed to describe the pattern forming process by a
sweeping interface upon drying the water-granule mixture. The model is shown to
produce labyrinthine patterns similar to those found in the experiment[Yamazaki
and Mizuguchi, J. Phys. Soc. Jpn. \textbf{69} (2000) 2387]. Upon changing the
initial granular density, resulting patterns undergo the percolation
transition, but estimated critical exponents are different from those of the
conventional percolation. Loopless structure of clusters in the patterns
produced by the sweeping dynamics seems to influence the nature of the
transition.Comment: 6 pages, 7 figure
- âŠ