26,851 research outputs found
Measuring Galactic Extinction: A Test
We test the recently published all-sky reddening map of Schlegel, Finkbeiner
& Davis (1998 [SFD]) using the extinction study of a region in the Taurus dark
cloud complex by Arce & Goodman (1999 [AG]). In their study, AG use four
different techniques to measure the amount and structure of the extinction
toward Taurus, and all four techniques agree very well. Thus we believe that
the AG results are a truthful representation of the extinction in the region
and can be used to test the reliability of the SFD reddening map. The results
of our test show that the SFD all-sky reddening map, which is based on data
from COBE/DIRBE and IRAS/ISSA, overestimates the reddening by a factor of 1.3
to 1.5 in regions of smooth extinction with A_V > 0.5 mag. In some regions of
steep extinction gradients the SFD map underestimates the reddening value,
probably due to its low spatial resolution. We expect that the astronomical
community will be using the SFD reddening map extensively. We offer this Letter
as a cautionary note about using the SFD map in regions of high extinction (A_V
> 0.5 mag), as it might not be giving accurate reddening values there.Comment: 14 pages (which include 2 pages of figures
Multigrid Monte Carlo with higher cycles in the Sine Gordon model
We study the dynamical critical behavior of multigrid Monte Carlo for the two
dimensional Sine Gordon model on lattices up to 128 x 128. Using piecewise
constant interpolation, we perform a W-cycle (gamma=2). We examine whether one
can reduce critical slowing down caused by decreasing acceptance rates on large
blocks by doing more work on coarser lattices. To this end, we choose a higher
cycle with gamma = 4. The results clearly demonstrate that critical slowing
down is not reduced in either case.Comment: 7 pages, 1 figure, whole paper including figure contained in ps-file,
DESY 93-00
Association of Infrastructure and Route Environment Factors with Cycling Injury Risk at Intersection and Non-Intersection Locations: A Case-Crossover Study of Britain.
This paper examines infrastructural and route environment correlates of cycling injury risk in Britain for commuters riding in the morning peak. The study uses a case-crossover design which controls for exposure. Control sites from modelled cyclist routes (matched on intersection status) were compared with sites where cyclists were injured. Conditional logistic regression for matched case-control groups was used to compare characteristics of control and injury sites. High streets (defined by clustering of retail premises) raised injury odds by 32%. Main (Class A or primary) roads were riskier than other road types, with injury odds twice that for residential roads. Wider roads, and those with lower gradients increased injury odds. Guard railing raised injury odds by 18%, and petrol stations or car parks by 43%. Bus lanes raised injury odds by 84%. As in other studies, there was a 'safety in numbers' effect from more cyclists. Contrary to other analysis, including two recent studies in London, we did not find a protective effect from cycle infrastructure and the presence of painted cycle lanes raised injury odds by 54%. At intersections, both standard and mini roundabouts were associated with injury odds several times higher than other intersections. Presence of traffic signals, with or without an Advanced Stop Line ('bike box'), had no impact on injury odds. For a cyclist on a main road, intersections with minor roads were riskier than intersections with other main roads. Typical cycling environments in Britain put cyclists at risk, and infrastructure must be improved, particularly on busy main roads, high streets, and bus routes
Systematic study of the PDC speckle structure for quantum imaging applications
Sub shot noise imaging of weak object by exploiting Parametric Down Converted
light represents a very interesting technological development. A precise
characterization of PDC speckle structure in dependence of pump beam parameters
is a fundamental tool for this application. In this paper we present a first
set of data addressed to this purpose
Dynamics of vortex dipoles in anisotropic Bose-Einstein condensates
We study the motion of a vortex dipole in a Bose-Einstein condensate confined
to an anisotropic trap. We focus on a system of ordinary differential equations
describing the vortices' motion, which is in turn a reduced model of the
Gross-Pitaevskii equation describing the condensate's motion. Using a sequence
of canonical changes of variables, we reduce the dimension and simplify the
equations of motion. We uncover two interesting regimes. Near a family of
periodic orbits known as guiding centers, we find that the dynamics is
essentially that of a pendulum coupled to a linear oscillator, leading to
stochastic reversals in the overall direction of rotation of the dipole. Near
the separatrix orbit in the isotropic system, we find other families of
periodic, quasi-periodic, and chaotic trajectories. In a neighborhood of the
guiding center orbits, we derive an explicit iterated map that simplifies the
problem further. Numerical calculations are used to illustrate the phenomena
discovered through the analysis. Using the results from the reduced system we
are able to construct complex periodic orbits in the original, partial
differential equation, mean-field model for Bose-Einstein condensates, which
corroborates the phenomenology observed in the reduced dynamical equations
Propagation of temporal entanglement
The equations that govern the temporal evolution of two photons in the
Schr{\"o}dinger picture are derived, taking into account the effects of loss,
group-velocity dispersion, temporal phase modulation, linear coupling among
different optical modes, and four-wave mixing. Inspired by the formalism, we
propose the concept of quantum temporal imaging, which uses dispersive elements
and temporal phase modulators to manipulate the temporal correlation of two
entangled photons. We also present the exact solution of a two-photon vector
soliton, in order to demonstrate the ease of use and intuitiveness of the
proposed formulation.Comment: 8 pages, 4 figure
Theoretical Analysis of Acceptance Rates in Multigrid Monte Carlo
We analyze the kinematics of multigrid Monte Carlo algorithms by
investigating acceptance rates for nonlocal Metropolis updates. With the help
of a simple criterion we can decide whether or not a multigrid algorithm will
have a chance to overcome critial slowing down for a given model. Our method is
introduced in the context of spin models. A multigrid Monte Carlo procedure for
nonabelian lattice gauge theory is described, and its kinematics is analyzed in
detail.Comment: 7 pages, no figures, (talk at LATTICE 92 in Amsterdam
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