2,814 research outputs found
Generalised action-angle coordinates defined on island chains
Straight-field-line coordinates are very useful for representing magnetic
fields in toroidally confined plasmas, but fundamental problems arise regarding
their definition in 3-D geometries because of the formation of islands and
chaotic field regions, ie non-integrability. In Hamiltonian dynamical systems
terms these coordinates are a form of action-angle variables, which are
normally defined only for integrable systems. In order to describe 3-D magnetic
field systems, a generalisation of this concept was proposed recently by the
present authors that unified the concepts of ghost surfaces and
quadratic-flux-minimising (QFMin) surfaces. This was based on a simple
canonical transformation generated by a change of variable , where and are poloidal and toroidal
angles, respectively, with a new poloidal angle chosen to give
pseudo-orbits that are a) straight when plotted in the plane and
b) QFMin pseudo-orbits in the transformed coordinate. These two requirements
ensure that the pseudo-orbits are also c) ghost pseudo-orbits. In the present
paper, it is demonstrated that these requirements do not \emph{uniquely}
specify the transformation owing to a relabelling symmetry. A variational
method of solution that removes this lack of uniqueness is proposed.Comment: 10 pages. Accepted by Plasma Physics and Controlled Fusion as part of
a cluster of refereed papers in a special issue containing papers arising
from the Joint International Stellarator & Heliotron Workshop and
Asia-Pacific Plasma Theory Conference, held in Canberra and Murramarang
Resort, Australia, 30 January - 3 February, 201
Portable dynamic fundus instrument
A portable diagnostic image analysis instrument is disclosed for retinal funduscopy in which an eye fundus image is optically processed by a lens system to a charge coupled device (CCD) which produces recordable and viewable output data and is simultaneously viewable on an electronic view finder. The fundus image is processed to develop a representation of the vessel or vessels from the output data
Preoperative neutrophil-lymphocyte ratio and outcome from coronary artery bypass grafting
Background: An elevated preoperative white blood cell count has been associated with a worse outcome after coronary artery bypass grafting (CABG). Leukocyte subtypes, and particularly the neutrophil-lymphocyte (N/L) ratio, may however, convey superior prognostic information. We hypothesized that the N/L ratio would predict the outcome of patients undergoing surgical revascularization. Methods: Baseline clinical details were obtained prospectively in 1938 patients undergoing CABG. The differential leukocyte was measured before surgery, and patients were followed-up 3.6 years later. The primary end point was all-cause mortality. Results: The preoperative N/L ratio was a powerful univariable predictor of mortality (hazard ratio [HR] 1.13 per unit, P 3.36). Conclusion: An elevated N/L ratio is associated with a poorer survival after CABG. This prognostic utility is independent of other recognized risk factors.Peer reviewedAuthor versio
The Evolution of Quasar CIV and SiIV Broad Absorption Lines Over Multi-Year Time Scales
We investigate the variability of CIV 1549A broad absorption line (BAL)
troughs over rest-frame time scales of up to ~7 yr in 14 quasars at redshifts
z>2.1. For 9 sources at sufficiently high redshift, we also compare CIV and
SiIV 1400A absorption variation. We compare shorter- and longer-term
variability using spectra from up to four different epochs per source and find
complex patterns of variation in the sample overall. The scatter in the change
of absorption equivalent width (EW), Delta EW, increases with the time between
observations. BALs do not, in general, strengthen or weaken monotonically, and
variation observed over shorter (<months) time scales is not predictive of
multi-year variation. We find no evidence for asymmetry in the distribution of
Delta EW that would indicate that BALs form and decay on different time scales,
and we constrain the typical BAL lifetime to be >~30 yr. The BAL absorption for
one source, LBQS 0022+0150, has weakened and may now be classified as a
mini-BAL. Another source, 1235+1453, shows evidence of variable, blue continuum
emission that is relatively unabsorbed by the BAL outflow. CIV and SiIV BAL
shape changes are related in at least some sources. Given their high
velocities, BAL outflows apparently traverse large spatial regions and may
interact with parsec-scale structures such as an obscuring torus. Assuming BAL
outflows are launched from a rotating accretion disk, notable azimuthal
symmetry is required in the outflow to explain the relatively small changes
observed in velocity structure over times up to 7 yr
The X-Ray Properties of the Optically Brightest Mini-BAL Quasars from the Sloan Digital Sky Survey
We have compiled a sample of 14 of the optically brightest radio-quiet
quasars (~~17.5 and ~~1.9) in the Sloan Digital Sky Survey
Data Release 5 quasar catalog that have C IV mini-BALs present in their
spectra. X-ray data for 12 of the objects were obtained via a Chandra snapshot
survey using ACIS-S, while data for the other two quasars were obtained from
archival XMM-Newton observations. Joint X-ray spectral analysis shows the
mini-BAL quasars have a similar average power-law photon index
() and level of intrinsic absorption () as non-BMB (neither BAL nor mini-BAL) quasars.
Mini-BAL quasars are more similar to non-BMB quasars than to BAL quasars in
their distribution of relative X-ray brightness (assessed with
). Relative colors indicate mild dust reddening in the
optical spectra of mini-BAL quasars. Significant correlations between
and UV absorption properties are confirmed for a sample
of 56 sources combining mini-BAL and BAL quasars with high signal-to-noise
ratio rest-frame UV spectra, which generally supports models in which X-ray
absorption is important in enabling driving of the UV absorption-line wind. We
also propose alternative parametrizations of the UV absorption properties of
mini-BAL and BAL quasars, which may better describe the broad absorption
troughs in some respects.Comment: ApJ accepted; 21 pages, 11 figures, and 9 table
The variability of warm absorbers in Active Galactic Nuclei
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.Includes bibliographical references (p. 171-184).This thesis presents three studies of warm (photoionized) absorber variability in Active Galactic Nuclei (AGN) using high-resolution X-ray spectra provided by the Chandra High Energy Transmission Grating (HETG). The first study is a single observation of the AGN MR 2251-178, which is known to have a highly variable warm absorber (WA). We find an unusually thin effective hydrogen column density along the line of sight compared to previous observations. Strong line emission without corresponding absorption indicates significant WA geometric structure. Strong absorption features in the spectrum are evidence of a highly-ionized, high-velocity outflow, which could be carrying a large amount of mass and energy out of the AGN. In the second study, we search for absorption lines variability in the well-studied WA of MCG -6-30-15. We find a significant anti-correlation over time between at least two ions, with suggestions of additional time variation in other ions. At least one line, the is - 2p resonance line of Mg XII, varies as a function of 2-10 keV continuum luminosity. Luminosity-driven ionization changes alone are insufficient to explain the observed variation.(cont.) Either multiple factors influence line strength on observable time scales, or the line of sight to the central source varies over time through a structured absorber. In the third study, we survey spectra from the HETG data archive. We model the normalized excess variance (NEV) spectrum of a varying WA and find that it does not explain high-energy (> 2 keV) spectral variation, nor does it generally fit NEV spectra at lower energies (< 2 keV). We also search through each spectrum at high resolution (AA = 0.01 A) for bins which vary more than expected due to normal Poissonian fluctuations. We find some evidence for such variation in the aggregate sample, though not in AGN individually. Our results show that WA structure is more complicated than pictured in contemporary models. Future high-resolution spectroscopic variability studies are certainly warranted. AGN models should eventually consider the effects of WA structure and the influence of continuum variation on the WA.by Robert R. Gibson.Ph.D
EFFECT OF INFARCT SIZE LIMITATION BY PROPRANOLOL ON VENTRICULAR ARRHYTHMIAS AFTER MYOCARDIAL INFARCTION
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75625/1/j.1749-6632.1982.tb55220.x.pd
Potential of novel dextran oligosaccharides as prebiotics for obesity management through in vitro experimentation
The energy-salvaging capacity of the gut microbiota from dietary ingredients has been proposed as a contributing factor for the development of obesity. This knowledge generated interest in the use of non-digestible dietary ingredients such as prebiotics to manipulate host energy homeostasis. In the present study, the in vitro response of obese human faecal microbiota to novel oligosaccharides was investigated. Dextrans of various molecular weights and degrees of branching were fermented with the faecal microbiota of healthy obese adults in pH-controlled batch cultures. Changes in bacterial populations were monitored using fluorescent in situ hybridisation and SCFA concentrations were analysed by HPLC. The rate of gas production and total volume of gas produced were also determined. In general, the novel dextrans and inulin increased the counts of bifidobacteria. Some of the dextrans were able to alter the composition of the obese human microbiota by increasing the counts of Bacteroides–Prevotella and decreasing those of Faecalibacterium prausnitzii and Ruminococcus bromii/R. flavefaciens. Considerable increases in SCFA concentrations were observed in response to all substrates. Gas production rates were similar during the fermentation of all dextrans, but significantly lower than those during the fermentation of inulin. Lower total gas production and shorter time to attain maximal gas production were observed during the fermentation of the linear 1 kDa dextran than during the fermentation of the other dextrans. The efficacy of bifidobacteria to ferment dextrans relied on the molecular weight and not on the degree of branching. In conclusion, there are no differences in the profiles between the obese and lean human faecal fermentations of dextrans
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