1,904 research outputs found
Flight research capabilities of the NASA/Army rotor systems research aircraft
A description is given of the capabilities and limitations of the Rotor Systems Research Aircraft (RSRA) that was demonstrated during the development contract, and assesses the expected research capabilities of the RSRA on delivery to the government
Design of recursive digital filters having specified phase and magnitude characteristics
A method for a computer-aided design of a class of optimum filters, having specifications in the frequency domain of both magnitude and phase, is described. The method, an extension to the work of Steiglitz, uses the Fletcher-Powell algorithm to minimize a weighted squared magnitude and phase criterion. Results using the algorithm for the design of filters having specified phase as well as specified magnitude and phase compromise are presented
RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete Sample of 1500 Bright X-ray Sources
We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAO VLA
Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightest X-ray
sources (>= 0.1 counts/s or ~1E-12 ergs/cm/cm/s in the 0.1-2.4 keV band) that
are also radio sources (S >= 2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalactic
sky with |b| > 15 degrees. and delta > -40 degrees. The sky density of NVSS
sources is low enough that they can be reliably identified with RBSC sources
having average rms positional uncertainties = 10 arcsec. We used the more
accurate radio positions to make reliable X-ray/radio/optical identifications
down to the POSS plate limits. We obtained optical spectra for many of the
bright identifications lacking published redshifts. The resulting X-ray/radio
sample is unique in its size (N ~ 1500 objects), composition (a mixture of
nearly normal galaxies, Seyfert galaxies, quasars, and clusters), and low
average redshift ( ~ 0.1).Comment: 35 LaTeX pages including 6 eps figures + 40 LaTeX page table2
(landscape) w/ AASTeX 5.0; accepted to ApJ
Automated optical identification of a large complete northern hemisphere sample of flat spectrum radio sources with S_6cm > 200 mJy
This paper describes the automated optical APM identification of radio
sources from the Jodrell Bank - VLA Astrometric Survey (JVAS), as used for the
search for distant radio-loud quasars. The sample has been used to investigate
possible relations between optical and radio properties of flat spectrum radio
sources. From the 915 sources in the sample, 756 have an optical APM
identification at a red (e) and/or blue (o) plate,resulting in an
identification fraction of 83% with a completeness and reliability of 98% and
99% respectively. About 20% are optically identified with extended APM objects
on the red plates, e.g. galaxies. However the distinction between galaxies and
quasars can not be done properly near the magnitude limit of the POSS-I plates.
The identification fraction appears to decrease from >90% for sources with a 5
GHz flux density of >1 Jy, to <80% for sources at 0.2 Jy. The identification
fraction, in particular that for unresolved quasars, is found to be lower for
sources with steeper radio spectra. In agreement with previous studies, we find
that the quasars at low radio flux density levels also tend to have fainter
optical magnitudes, although there is a large spread. In addition, objects with
a steep radio-to-optical spectral index are found to be mainly highly polarised
quasars, supporting the idea that in these objects the polarised synchrotron
component is more prominent. It is shown that the large spread in
radio-to-optical spectral index is possibly caused by source to source
variations in the Doppler boosting of the synchrotron component [Abridged].Comment: LaTex, 17 pages, 5 gif figures, 4 tables. Accepted for publication in
MNRAS. High resolution figures can be found at http://www.roe.ac.uk/~ignas
Atherosclerotic carotid plaque composition: a 3T and 7T MRI-histology correlation study
Background and Purpose
Carotid artery atherosclerotic plaque composition may influence plaque stability and risk of
thromboembolic events, and non-invasive plaque imaging may therefore permit risk
stratification for clinical management. Plaque composition was compared using non-invasive
in-vivo (3T) and ex-vivo (7T) MRI and histopathological examination.
Methods
Thirty three endarterectomy cross sections, from 13 patients, were studied. The datasets
consisted of in-vivo 3T MRI, ex-vivo 7T MRI and histopathology. Semi-automated
segmentation methods were used to measure areas of different plaque components. Bland-
Altman plots and mean difference with 95% confidence interval were carried out.
Results
There was general quantitative agreement between areas derived from semi-automated
segmentation of MRI data and histology measurements. The mean differences and 95%
confidence bounds in the relative to total plaque area between 3T versus Histology were:
fibrous tissue 4.99 % (-4.56 to 14.56), lipid-rich/necrotic core (LR/NC) with haemorrhage -
1.81% (-14.11 to 10.48), LR/NC without haemorrhage -2.43% (-13.04 to 8.17), and
calcification -3.18% (-11.55 to 5.18). The mean differences and 95% confidence bounds in
the relative to total plaque area between 7T and histology were: fibrous tissue 3.17 % (-3.17
to 9.52), LR/NC with haemorrhage -0.55% (-9.06 to 7.95), LR/NC without haemorrhage -
12.62% (-19.8 to -5.45), and calcification -2.43% (-9.97 to 4.73).
Conclusions
This study provides evidence that semi-automated segmentation of 3T/7T MRI techniques
can help to determine atherosclerotic plaque composition. In particular, the high resolution of
ex-vivo 7T data was able to highlight greater detail in the atherosclerotic plaque composition.
High field MRI may therefore have advantages for in vivo carotid plaque MR imaging
Radio continuum and far-infrared emission from the galaxies in the Eridanus group
The Eridanus galaxies follow the well-known radio-FIR correlation. Majority
(70%) of these galaxies have their star formation rates below that of the Milky
Way. The galaxies having a significant excess of radio emission are identified
as low luminosity AGNs based on their radio morphologies obtained from the GMRT
observations. There are no powerful AGNs (L{20cm} > 10^{23} W Hz^{-1}) in the
group. The two most far-infrared and radio luminous galaxies in the group have
optical and HI morphologies suggestive of recent tidal interactions. The
Eridanus group also has two far-infrared luminous but radio-deficient galaxies.
It is believed that these galaxies are observed within a few Myr of the onset
of an intense star formation episode after being quiescent for at least a 100
Myr. The upper end of the radio luminosity distribution of the Eridanus
galaxies (L_{20cm} ~ 10^{22} W Hz^{-1}) is consistent with that of the field
galaxies, other groups, and late-type galaxies in nearby clusters.Comment: 16 pages; Accepted for publication in Journal of Astroph. & Astron.
March, 200
Polarization dependence of x-ray absorption spectra in Na_xCoO_2
In order to shed light on the electronic structure of Na_xCoO_2, and
motivated by recent Co L-edge X-ray absorption spectra (XAS) experiments with
polarized light, we calculate the electronic spectrum of a CoO_6 cluster
including all interactions between 3d orbitals. We obtain the ground state for
two electronic occupations in the cluster that correspond nominally to all O in
the O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited states
obtained by promotion of a Co 2p electron to a 3d electron, and the
corresponding matrix elements are calculated. A fit of the observed
experimental spectra is good and points out a large Co-O covalency and cubic
crystal field effects, that result in low spin Co 3d configurations. Our
results indicate that the effective hopping between different Co atoms plays a
major role in determining the symmetry of the ground state in the lattice.
Remaining quantitative discrepancies with the XAS experiments are expected to
come from composition effects of itineracy in the ground and excited states.Comment: 10 pages, 4 figure
Decay process accelerated by tunneling in its very early stage
We examine a fast decay process that arises in the transition period between
the Gaussian and exponential decay processes in quantum decay systems. It is
usually expected that the decay is decelerated by a confinement potential
barrier. However, we find a case where the decay in the transition period is
accelerated by tunneling through a confinement potential barrier. We show that
the acceleration gives rise to an appreciable effect on the time evolution of
the nonescape probability of the decay system.Comment: 4 pages, 6 figures; accepted for publication in Phys. Rev.
Folding model analysis of alpha radioactivity
Radioactive decay of nuclei via emission of particles has been
studied theoretically in the framework of a superasymmetric fission model using
the double folding (DF) procedure for obtaining the -nucleus
interaction potential. The DF nuclear potential has been obtained by folding in
the density distribution functions of the nucleus and the daughter
nucleus with a realistic effective interaction. The M3Y effective interaction
has been used for calculating the nuclear interaction potential which has been
supplemented by a zero-range pseudo-potential for exchange along with the
density dependence. The nuclear microscopic -nucleus potential thus
obtained has been used along with the Coulomb interaction potential to
calculate the action integral within the WKB approximation. This subsequently
yields microscopic calculations for the half lives of decays of
nuclei. The density dependence and the exchange effects have not been found to
be very significant. These calculations provide reasonable estimates for the
lifetimes of radioactivity of nuclei.Comment: 7 pages including 1 figur
A model for single electron decays from a strongly isolated quantum dot
Recent measurements of electron escape from a non-equilibrium charged quantum
dot are interpreted within a 2D separable model. The confining potential is
derived from 3D self-consistent Poisson-Thomas-Fermi calculations. It is found
that the sequence of decay lifetimes provides a sensitive test of the confining
potential and its dependence on electron occupation.Comment: 9 pages, 10 figure
- …