1,809 research outputs found
A Fourier-Based Algorithm for Modelling Aberrations in HETE-2's Imaging System
The High-Energy Transient Explorer (HETE-2), launched in October 2000, is a
satellite experiment dedicated to the study of gamma-ray bursts in a very wide
energy range from soft X-ray to gamma-ray wavelengths. The intermediate X-ray
range (2-30keV) is covered by the Wide-field X-ray Monitor WXM, a coded
aperture imager. In this article, an algorithm for reconstructing the positions
of gamma-ray bursts is described, which is capable of correcting systematic
aberrations to approximately 1 arcmin throughout the field of view.
Functionality and performance of this algorithm have been validated using data
from Monte Carlo simulations as well as from astrometric observations of the
X-ray source Scorpius X-1.Comment: 14 pages, 9 figures, 2 tables; Nucl.Instr.Meth., in pres
Logarithmic corrections in the free energy of monomer-dimer model on plane lattices with free boundaries
Using exact computations we study the classical hard-core monomer-dimer
models on m x n plane lattice strips with free boundaries. For an arbitrary
number v of monomers (or vacancies), we found a logarithmic correction term in
the finite-size correction of the free energy. The coefficient of the
logarithmic correction term depends on the number of monomers present (v) and
the parity of the width n of the lattice strip: the coefficient equals to v
when n is odd, and v/2 when n is even. The results are generalizations of the
previous results for a single monomer in an otherwise fully packed lattice of
dimers.Comment: 4 pages, 2 figure
Dual Monte Carlo and Cluster Algorithms
We discuss the development of cluster algorithms from the viewpoint of
probability theory and not from the usual viewpoint of a particular model. By
using the perspective of probability theory, we detail the nature of a cluster
algorithm, make explicit the assumptions embodied in all clusters of which we
are aware, and define the construction of free cluster algorithms. We also
illustrate these procedures by rederiving the Swendsen-Wang algorithm,
presenting the details of the loop algorithm for a worldline simulation of a
quantum 1/2 model, and proposing a free cluster version of the
Swendsen-Wang replica method for the random Ising model. How the principle of
maximum entropy might be used to aid the construction of cluster algorithms is
also discussed.Comment: 25 pages, 4 figures, to appear in Phys.Rev.
Computational Fluid Dynamics in Small Airway Models of the Human Lung
The promise of gene replacement therapy for cystic fibrosis, the administration of drugs via inhalation therapy, and die deposition location of man-made airborne particulates all involve a more complete understanding of the fluid dynamics in the human lung. Flow in the larger airways may be measured through life-sized models directly, but the airways in the peripheral lung are too small and the flows are too complex to be studied in this manner. Computational models can be developed which will accurately represent both the geometric nature of the central airways and the fluid dynamics with in them. Two-dimensional and three-dimensional models of central lung airway bifurcations were developed based on morphometry. These models were used as the spatial basis upon which the differential equations that describe incompressible flow, the Navier Stokes equations, are solved. Flow solutions have been computed at Reynolds numbers from 1000 down to 100. Solutions for single and double bifurcations agree with the experimental data for flow in a branching tube. These studies are being extended to multiple bifurcations in three dimensions
New Lower Bounds on the Self-Avoiding-Walk Connective Constant
We give an elementary new method for obtaining rigorous lower bounds on the
connective constant for self-avoiding walks on the hypercubic lattice .
The method is based on loop erasure and restoration, and does not require exact
enumeration data. Our bounds are best for high , and in fact agree with the
first four terms of the expansion for the connective constant. The bounds
are the best to date for dimensions , but do not produce good results
in two dimensions. For , respectively, our lower bound is within
2.4\%, 0.43\%, 0.12\%, 0.044\% of the value estimated by series extrapolation.Comment: 35 pages, 388480 bytes Postscript, NYU-TH-93/02/0
Weighted distances in scale-free preferential attachment models
We study three preferential attachment models where the parameters are such
that the asymptotic degree distribution has infinite variance. Every edge is
equipped with a non-negative i.i.d. weight. We study the weighted distance
between two vertices chosen uniformly at random, the typical weighted distance,
and the number of edges on this path, the typical hopcount. We prove that there
are precisely two universality classes of weight distributions, called the
explosive and conservative class. In the explosive class, we show that the
typical weighted distance converges in distribution to the sum of two i.i.d.
finite random variables. In the conservative class, we prove that the typical
weighted distance tends to infinity, and we give an explicit expression for the
main growth term, as well as for the hopcount. Under a mild assumption on the
weight distribution the fluctuations around the main term are tight.Comment: Revised version, results are unchanged. 30 pages, 1 figure. To appear
in Random Structures and Algorithm
Searching for the in-plane Galactic bar and ring in DENIS
New evidence for a long thin Galactic bar (in contradistinction to the
bulge), as well as for the existence of the ring and the truncation of the
inner disc, are sought in the DENIS survey. First, we examine DENIS and Two
Micron Galactic Survey star counts for the characteristic signatures of an
in-plane bar and ring. The star counts in the plane for 30 deg.>l>-30 deg. are
shown to be highly asymmetric with considerably more sources at positive than
at negative longitudes. At |b|\approx 1.5 deg., however, the counts are nearly
symmetric. Therefore, the asymmetry is not due to the disc, which is shown to
have an inner truncation, or to the bulge, so there has to be another major
component in the inner Galaxy that is causing the asymmetries. This component
provides up to 50% of the detected sources in the plane between the bulge and
l=27 deg. or l=-14 deg. This component is shown to be consistent with an
in-plane bar with a position angle of 40 deg. and half-length of 3.9 kpc.
However, there is also a major peak in the counts at l=-22 deg., which
coincides with the tangential point of the so-called 3 kpc arm. This is shown
to be most probably a ring or a pseudo-ring. The extinction in the plane is
also shown to be asymmetric with more extinction at negative than at positive
longitudes. For l<8 deg. the extinction is shown to be slightly tilted with
respect to b=0 deg. in the same manner as the HI disc. We conclude that the
Galaxy is a fairly typical ringed barred spiral galaxy.Comment: 15 pages, 10 figures, accepted in A&
Producing the docile body: analysing Local Area Under-performance Inspection (LAUI)
Sir Michael Wilshaw, the head of the Office for Standards in Education (OfSTED), declared a 'new wave' of Local Area Under-performance Inspections (LAUI) of schools 'denying children the standard of education they deserve'. This paper examines how the threat of LAUI played out over three mathematics lessons taught by a teacher in her first year in the profession. A Foucauldian approach is mobilised with regard to disciplinary power and 'docile bodies'. The paper argues that, in the case in point, LAUI was a tool mediating performative conditions and, ultimately, the docile body. The paper will be of concern to policy sociologists, teachers, school leaders, and those interested in school inspection
OB Stars in the Solar Neighborhood I: Analysis of their Spatial Distribution
We present a newly-developed, three-dimensional spatial classification
method, designed to analyze the spatial distribution of early type stars within
the 1 kpc sphere around the Sun. We propose a distribution model formed by two
intersecting disks -the Gould Belt (GB) and the Local Galactic Disk (LGD)-
defined by their fundamental geometric parameters. Then, using a sample of
about 550 stars of spectral types earlier than B6 and luminosity classes
between III and V, with precise photometric distances of less than 1 kpc, we
estimate for some spectral groups the parameters of our model, as well as
single membership probabilities of GB and LGD stars, thus drawing a picture of
the spatial distribution of young stars in the vicinity of the Sun.Comment: 28 pages including 9 Postscript figures, one of them in color.
Accepted for publication in The Astronomical Journal, 30 January 200
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