118,666 research outputs found
Production of Millisecond Dips in Sco X-1 Count Rates by Dead Time Effects
Chang et al. (2006) reported millisecond duration dips in the X-ray intensity
of Sco X-1 and attributed them to occultations of the source by small
trans-Neptunian objects (TNOs). We have found multiple lines of evidence that
these dips are not astronomical in origin, but rather the result of high-energy
charged particle events in the RXTE PCA detectors. Our analysis of the RXTE
data indicates that at most 10% of the observed dips in Sco X-1 could be due to
occultations by TNOs, and, furthermore, we find no positive or supporting
evidence for any of them being due to TNOs. We therefore believe that it is a
mistake to conclude that any TNOs have been detected via occultation of Sco
X-1.Comment: Submitted to ApJ; uses emulateapj.cls, 8 pages with 8 figure
Local quantum critical point in the pseudogap Anderson model: finite-T dynamics and omega/T scaling
The pseudogap Anderson impurity model is a paradigm for locally critical
quantum phase transitions. Within the framework of the local moment approach we
study its finite-T dynamics, as embodied in the single-particle spectrum, in
the vicinity of the symmetric quantum critical point (QCP) separating
generalized Fermi-liquid (Kondo screened) and local moment phases. The scaling
spectra in both phases, and at the QCP itself, are obtained analytically. A key
result is that pure omega/T-scaling obtains at the QCP, where the Kondo
resonance has just collapsed. The connection between the scaling spectra in
either phase and that at the QCP is explored in detail.Comment: 12 pages, 7 figure
A bioeconomic model for determining the optimal response to a new weed incursion in Australian cropping systems
Invasions by non-indigenous plant species pose serious economic threats to Australian agricultural industries. When an invasion is discovered a decision has to be made as to whether to attempt to eradicate it, contain it or do nothing. These decisions should be based on long term benefits and costs. This paper describes a bioeconomic simulation framework with a mathematical model representing weed spread linked to a dynamic programming model to provide a means of determining the economically optimal weed management strategies over time. The modelling framework is used to evaluate case study invasive weed control problems in the Australian grains industry.weeds, incursion, bioeconomic model, Crop Production/Industries, Research Methods/ Statistical Methods,
The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales
We decipher intrinsic three-dimensional shape distributions of molecular
clouds, cloud cores, Bok globules, and condensations using recently compiled
catalogues of observed axis ratios for these objects mapped in carbon monoxide,
ammonia, through optical selection, or in continuum dust emission. We apply
statistical techniques to compare assumed intrinsic axis ratio distributions
with observed projected axis ratio distributions. Intrinsically triaxial shapes
produce projected distributions which agree with observations. Molecular clouds
mapped in CO are intrinsically triaxial but more nearly prolate than
oblate, while the smaller cloud cores, Bok globules, and condensations are also
intrinsically triaxial but more nearly oblate than prolate.Comment: 12 pages, 11 figures. Version with color figures can be found at
http://www.astro.uwo.ca/~cjones/ or http://www.astro.uwo.ca/~basu/. To appear
in ApJ, 10 April 2002, v. 569, no.
Macroscopic and microscopic studies of electrical properties of very thin silicon dioxide subject to electrical stress
The electrical characteristics of various size tunnel switch diode devices, composed of Al/SiO2/n-Si/p+-Si layers, which operate with a range of parameters (such as current densities in excess of 104 A/cm2) that stress the oxide layer far beyond the levels used in typical thin oxide metal-oxide semiconductor research have been examined. It is found that the first time a large current and electric field are applied to the device, a "forming" process enhances transport through the oxide in the vicinity of the edges of the gate electrode, but the oxide still retains its integrity as a tunnel barrier. The device operation is relatively stable to stresses of greater than 107 C/cm2 areally averaged, time-integrated charge injection. Duplication and characterization of these modified oxide tunneling properties was attempted using scanning tunneling microscopy (STM) to stress and probe the oxide. Electrical stressing with the STM tip creates regions of reduced conductivity, possibly resulting from trapped charge in the oxide. Lateral variations in the conductivity of the unstressed oxide over regions roughly 20–50 nm across were also found
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