9,532 research outputs found
Improved bounds for the number of forests and acyclic orientations in the square lattice
In a recent paper Merino and Welsh (1999) studied several counting problems on the square lattice . The authors gave the following bounds for the asymptotics of , the number of forests of , and , the number of acyclic orientations of : and .
In this paper we improve these bounds as follows: and . We obtain this by developing a method for computing the Tutte polynomial of the square lattice and other related graphs based on transfer matrices
Inductive and Electrostatic Acceleration in Relativistic Jet-Plasma Interactions
We report on the observation of rapid particle acceleration in numerical
simulations of relativistic jet-plasma interactions and discuss the underlying
mechanisms. The dynamics of a charge-neutral, narrow, electron-positron jet
propagating through an unmagnetized electron-ion plasma was investigated using
a three-dimensional, electromagnetic, particle-in-cell computer code. The
interaction excited magnetic filamentation as well as electrostatic plasma
instabilities. In some cases, the longitudinal electric fields generated
inductively and electrostatically reached the cold plasma wave-breaking limit,
and the longitudinal momentum of about half the positrons increased by 50% with
a maximum gain exceeding a factor of 2 during the simulation period. Particle
acceleration via these mechanisms occurred when the criteria for Weibel
instability were satisfied.Comment: Revised for Phys. Rev. Lett. Please see publised version for best
graphic
Universality and properties of neutron star type I critical collapses
We study the neutron star axisymmetric critical solution previously found in
the numerical studies of neutron star mergers. Using neutron star-like initial
data and performing similar merger simulations, we demonstrate that the
solution is indeed a semi-attractor on the threshold plane separating the basin
of a neutron star and the basin of a black hole in the solution space of the
Einstein equations. In order to explore the extent of the attraction basin of
the neutron star semiattractor, we construct initial data phase spaces for
these neutron star-like initial data. From these phase spaces, we also observe
several interesting dynamical scenarios where the merged object is supported
from prompt collapse. The properties of the critical index of the solution, in
particular, its dependence on conserved quantities, are then studied. From the
study, it is found that a family of neutron star semi-attractors exist that can
be classified by both their rest masses and ADM masses.Comment: 13 pages, 12 figures, 1 new reference adde
Development of the Lymphoedema Genito-Urinary Cancer Questionnaire
The aim of this study was to develop a patient self-report tool to
detect symptoms of genital and lower limb lymphoedema in male
survivors of genitourinary cancer. The study incorporated the views
of patients and subject specialists (lymphoedema and urology) in the
design of a patient questionnaire based on the literature. Views on
comprehensiveness, relevance of content, ease of understanding and
perceived acceptability to patients were collated. The findings informed
the development of the next iteration of the questionnaire. The overall
view of participants was that the development and application of
such a tool was of great clinical value and the Lymphoedema Genito-
Urinary Cancer Questionnaire (LGUCQ) has significant potential for
further development as a research tool to inform prevalence of this
under-reported condition
Robustness of the Blandford-Znajek mechanism
The Blandford-Znajek mechanism has long been regarded as a key ingredient in
models attempting to explain powerful jets in AGNs, quasars, blazzars etc. In
such mechanism, energy is extracted from a rotating black hole and dissipated
at a load at far distances. In the current work we examine the behaviour of the
BZ mechanism with respect to different boundary conditions, revealing the
mechanism robustness upon variation of these conditions. Consequently, this
work closes a gap in our understanding of this important scenario.Comment: 7 pages, accepted in CQ
Approaching the event horizon: 1.3mm VLBI of SgrA*
Advances in VLBI instrumentation now allow wideband recording that
significantly increases the sensitivity of short wavelength VLBI observations.
Observations of the super-massive black hole candidate at the center of the
Milky Way, SgrA*, with short wavelength VLBI reduces the scattering effects of
the intervening interstellar medium, allowing observations with angular
resolution comparable to the apparent size of the event horizon of the putative
black hole. Observations in April 2007 at a wavelength of 1.3mm on a three
station VLBI array have now confirmed structure in SgrA* on scales of just a
few Schwarzschild radii. When modeled as a circular Gaussian, the fitted
diameter of SgrA* is 37 micro arcsec (+16,-10; 3-sigma), which is smaller than
the expected apparent size of the event horizon of the Galactic Center black
hole. These observations demonstrate that mm/sub-mm VLBI is poised to open a
new window onto the study of black hole physics via high angular resolution
observations of the Galactic Center.Comment: 6 pages, 4 figures, Proceedings for "The Universe under the
Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be
published in Journal of Physics: Conference Series by Institute of Physics
Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.
A four-element end-fire microphone array for acoustic measurements in wind tunnels
A prototype four-element end-fire microphone array was designed and built for evaluation as a directional acoustic receiver for use in large wind tunnels. The microphone signals were digitized, time delayed, summed, and reconverted to analog form in such a way as to create a directional response with the main lobe along the array axis. The measured array directivity agrees with theoretical predictions confirming the circuit design of the electronic control module. The array with 0.15 m (0.5 ft) microphone spacing rejected reverberations and background noise in the Ames 40- by 80-foot wind tunnel by 5 to 12 db for frequencies above 400 Hz
Space Weathering of Ordinary Chondrite Parent Bodies, Its Impact on the Method of Distinguishing H, L, and LL Types and Implications for Itokawa Samples Returned by the Hayabusa Mission
As the most abundance meteorites in our collections, ordinary chondrites potentially have very important implications on the origin and formation of our Solar System. In order to map the distribution of ordinary chondrite-like asteroids through remote sensing, the space weathering effects of ordinary chondrite parent bodies must be addressed through experiments and modeling. Of particular importance is the impact on distinguishing different types (H/L/LL) of ordinary chondrites. In addition, samples of asteroid Itokawa returned by the Hayabusa spacecraft may re~ veal the mechanism of space weathering on an LLchondrite parent body. Results of space weathering simulations on ordinary chondrites and implications for Itokawa samples are presented here
Beyond XSPEC: Towards Highly Configurable Analysis
We present a quantitative comparison between software features of the defacto
standard X-ray spectral analysis tool, XSPEC, and ISIS, the Interactive
Spectral Interpretation System. Our emphasis is on customized analysis, with
ISIS offered as a strong example of configurable software. While noting that
XSPEC has been of immense value to astronomers, and that its scientific core is
moderately extensible--most commonly via the inclusion of user contributed
"local models"--we identify a series of limitations with its use beyond
conventional spectral modeling. We argue that from the viewpoint of the
astronomical user, the XSPEC internal structure presents a Black Box Problem,
with many of its important features hidden from the top-level interface, thus
discouraging user customization. Drawing from examples in custom modeling,
numerical analysis, parallel computation, visualization, data management, and
automated code generation, we show how a numerically scriptable, modular, and
extensible analysis platform such as ISIS facilitates many forms of advanced
astrophysical inquiry.Comment: Accepted by PASP, for July 2008 (15 pages
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