347,855 research outputs found
A model for the dynamics of extensible semiflexible polymers
We present a model for semiflexible polymers in Hamiltonian formulation which
interpolates between a Rouse chain and worm-like chain. Both models are
realized as limits for the parameters. The model parameters can also be chosen
to match the experimental force-extension curve for double-stranded DNA. Near
the ground state of the Hamiltonian, the eigenvalues for the longitudinal
(stretching) and the transversal (bending) modes of a chain with N springs,
indexed by p, scale as lambda_lp ~ (p/N)^2 and lambda_tp ~ p^2(p-1)^2/N^4
respectively for small p. We also show that the associated decay times tau_p ~
(N/p)^4 will not be observed if they exceed the orientational time scale tau_r
~ N^3 for an equally-long rigid rod, as the driven decay is then washed out by
diffusive motion.Comment: 28 pages, 2 figure
Compression of sub-relativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffraction
We demonstrate compression of 95 keV, space-charge-dominated electron bunches
to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are
of sufficient quality to capture a diffraction pattern with a single shot,
which we demonstrate by a diffraction experiment on a polycrystalline gold
foil. Compression is realized by means of velocity bunching as a result of a
velocity chirp, induced by the oscillatory longitudinal electric field of a 3
GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs
Noncommutative curves and noncommutative surfaces
In this survey article we describe some geometric results in the theory of
noncommutative rings and, more generally, in the theory of abelian categories.
Roughly speaking and by analogy with the commutative situation, the category
of graded modules modulo torsion over a noncommutative graded ring of
quadratic, respectively cubic growth should be thought of as the noncommutative
analogue of a projective curve, respectively surface. This intuition has lead
to a remarkable number of nontrivial insights and results in noncommutative
algebra. Indeed, the problem of classifying noncommutative curves (and
noncommutative graded rings of quadratic growth) can be regarded as settled.
Despite the fact that no classification of noncommutative surfaces is in sight,
a rich body of nontrivial examples and techniques, including blowing up and
down, has been developed.Comment: Suggestions by many people (in particular Haynes Miller and Dennis
Keeler) have been incorporated. The formulation of some results has been
improve
A model-independent analysis of the variability of GRS 1915+105
We analyzed 163 observations of the microquasar GRS 1915+105 made with the
Rossi X-ray Timing Explorer (RXTE) in the period 1996-1997. For each
observation, we produced light curves and color-color diagrams. We classified
the observations in 12 separate classes, based on their count rate and color
characteristics. From the analysis of these classes, we reduced the variability
of the source to transitions between three basic states: a hard state
corresponding to the non-observability of the innermost parts of the accretion
disk, and two softer states with a fully observable disk. These two soft states
represent different temperatures of the accretion disk, related to different
local values of the accretion rate. The transitions between these states can be
extremely fast. The source moves between these three states following certain
patterns and avoiding others, giving rise to a relatively large but limited
number of variability classes. These results are the first step towards a
linking of the properties of this exceptional source with standard black-hole
systems and with accretion disk models.Comment: Accepted for publication in Astronomy & Astrophysics, 2000 January
6t
Finding faint HI structure in and around galaxies: scraping the barrel
Soon to be operational HI survey instruments such as APERTIF and ASKAP will
produce large datasets. These surveys will provide information about the HI in
and around hundreds of galaxies with a typical signal-to-noise ratio of
10 in the inner regions and 1 in the outer regions. In addition, such
surveys will make it possible to probe faint HI structures, typically located
in the vicinity of galaxies, such as extra-planar-gas, tails and filaments.
These structures are crucial for understanding galaxy evolution, particularly
when they are studied in relation to the local environment. Our aim is to find
optimized kernels for the discovery of faint and morphologically complex HI
structures. Therefore, using HI data from a variety of galaxies, we explore
state-of-the-art filtering algorithms. We show that the intensity-driven
gradient filter, due to its adaptive characteristics, is the optimal choice. In
fact, this filter requires only minimal tuning of the input parameters to
enhance the signal-to-noise ratio of faint components. In addition, it does not
degrade the resolution of the high signal-to-noise component of a source. The
filtering process must be fast and be embedded in an interactive visualization
tool in order to support fast inspection of a large number of sources. To
achieve such interactive exploration, we implemented a multi-core CPU (OpenMP)
and a GPU (OpenGL) version of this filter in a 3D visualization environment
().Comment: 17 pages, 9 figures, 4 tables. Astronomy and Computing, accepte
Vocal behaviour of Norwegian killer whales, Orcinus orca, during carousel and seiner foraging on spring-spawning herring
3-D interactive visualisation tools for HI spectral line imaging
Upcoming HI surveys will deliver such large datasets that automated
processing using the full 3-D information to find and characterize HI objects
is unavoidable. Full 3-D visualization is an essential tool for enabling
qualitative and quantitative inspection and analysis of the 3-D data, which is
often complex in nature. Here we present , an open-source
extension of 3DSlicer, a multi-platform open source software package for
visualization and medical image processing, which we developed for the
inspection and analysis of HI spectral line data. We describe its initial
capabilities, including 3-D filtering, 3-D selection and comparative modelling
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