5,451 research outputs found
Strong lensing optical depths in a \LambdaCDM universe
We investigate strong gravitational lensing in the concordance CDM
cosmology by carrying out ray-tracing along past light cones through the
Millennium Simulation, the largest simulation of cosmic structure formation
ever carried out. We extend previous ray-tracing methods in order to take full
advantage of the large volume and the excellent spatial and mass resolution of
the simulation. As a function of source redshift we evaluate the probability
that an image will be highly magnified, will be highly elongated or will be one
of a set of multiple images. We show that such strong lensing events can almost
always be traced to a single dominant lensing object and we study the mass and
redshift distribution of these primary lenses. We fit analytic models to the
simulated dark halos in order to study how our optical depth measurements are
affected by the limited resolution of the simulation and of the lensing planes
that we construct from it. We conclude that such effects lead us to
underestimate total strong-lensing cross sections by about 15 percent. This is
smaller than the effects expected from our neglect of the baryonic components
of galaxies. Finally we investigate whether strong lensing is enhanced by
material in front of or behind the primary lens. Although strong lensing
lines-of-sight are indeed biased towards higher than average mean densities,
this additional matter typically contributes only a few percent of the total
surface density.Comment: version accepted for publicatio
Atomic Effective Pseudopotentials for Semiconductors
We derive an analytic connection between the screened self-consistent
effective potential from density functional theory (DFT) and atomic effective
pseudopotentials (AEPs). The motivation to derive AEPs is to address structures
with thousands to hundred thousand atoms, as given in most nanostructures. The
use of AEPs allows to bypass a self-consistent procedure and to address
eigenstates around a certain region of the spectrum (e.g., around the band
gap). The bulk AEP construction requires two simple DFT calculations of
slightly deformed elongated cells. The ensuing AEPs are given on a fine
reciprocal space grid, including the small reciprocal vector components, are
free of parameters, and involve no fitting procedure. We further show how to
connect the AEPs of different bulk materials, which is necessary to obtain
accurate band offsets. We derive a total of 20 AEPs for III-V, II-VI and group
IV semiconductors and demonstrate their accuracy and transferability by
comparison to DFT calculations of strained bulk structures, quantum wells with
varying thickness, and semiconductor alloys.Comment: 10 pages, 5 figures, submitted to PR
On the nonlinear mapping of an ocean wave spectrum into a synthetic aperture radar image spectrum and its inversion
A new, closed nonlinear integral transformation relation is derived describing the mapping of a two-dimensional ocean wave spectrum into a synthetic aperture radar (SAR) image spectrum. The general integral relation is expanded in a power series with respect to orders of nonlinearity and velocity bunching. The individual terms of the series can be readily computed using fast Fourier transforms. The convergence of the series is rapid. The series expansion is also useful in identifying the different contributions to the net imaging process, consisting of the real aperture radar (RAR) cross-section modulation, the nonlinear motion (velocity bunching) effects, and their various interaction products. The lowest term of the expansion with respect to nonlinearity order yields a simple quasi-linear approximate mapping relation consisting of the standard linear SAR modulation expression multiplied by an additional nonlinear Gaussian azimuthal cutoff factor. The cutoff scale is given by the rms azimuthal (velocity bunching) displacement. The same cutoff factor applies to all terms of the power series expansion. The nonlinear mapping relation is inverted using a standard first-guess wave spectrum as regularization term. This is needed to overcome the basic 180° mapping ambiguity and the loss of information beyond the azimuthal cutoff. The inversion is solved numerically using an iteration technique based on the successive application of the explicit solution for the quasi-linear mapping approximation, with interposed corrections invoking the full nonlinear mapping expression. A straightforward application of this technique, however, generally yields unrealistic discontinuities of the best fit wave spectrum in the transition region separating the low azimuthal wave number domain, in which useful SAR information is available and the wave spectrum is modified, from the high azimuthal wave number region beyond the azimuthal cutoff, where the first-guess wave spectrum is retained. This difficulty is overcome by applying a two-step inversion procedure. In the first step the energy level of the wave spectrum is adjusted, and the wave number plane rotated and rescaled, without altering the shape of the spectrum. Using the resulting globally fitted spectrum as the new first-guess input spectrum, the original inversion method is then applied without further constraints in a second step to obtain a final fine-scale optimized spectrum. The forward mapping relation and inversion algorithms are illustrated for three Seasat cases representing different wave conditions corresponding to weakly, moderately, and strongly nonlinear imaging conditions
A General Transfer-Function Approach to Noise Filtering in Open-Loop Quantum Control
We present a general transfer-function approach to noise filtering in
open-loop Hamiltonian engineering protocols for open quantum systems. We show
how to identify a computationally tractable set of fundamental filter
functions, out of which arbitrary transfer filter functions may be assembled up
to arbitrary high order in principle. Besides avoiding the infinite recursive
hierarchy of filter functions that arises in general control scenarios, this
fundamental filter-functions set suffices to characterize the error suppression
capabilities of the control protocol in both the time and frequency domain. We
prove that the resulting notion of filtering order reveals conceptually
distinct, albeit complementary, features of the controlled dynamics as compared
to the order of error cancellation, traditionally defined in the Magnus sense.
Examples and implications are discussed.Comment: Paper plus supplementary material. 10 pages, 1 figure. Unnumbered
equation between 2 and 3 corrected. Results are unchange
Asteroid Models from Multiple Data Sources
In the past decade, hundreds of asteroid shape models have been derived using
the lightcurve inversion method. At the same time, a new framework of 3-D shape
modeling based on the combined analysis of widely different data sources such
as optical lightcurves, disk-resolved images, stellar occultation timings,
mid-infrared thermal radiometry, optical interferometry, and radar
delay-Doppler data, has been developed. This multi-data approach allows the
determination of most of the physical and surface properties of asteroids in a
single, coherent inversion, with spectacular results. We review the main
results of asteroid lightcurve inversion and also recent advances in multi-data
modeling. We show that models based on remote sensing data were confirmed by
spacecraft encounters with asteroids, and we discuss how the multiplication of
highly detailed 3-D models will help to refine our general knowledge of the
asteroid population. The physical and surface properties of asteroids, i.e.,
their spin, 3-D shape, density, thermal inertia, surface roughness, are among
the least known of all asteroid properties. Apart for the albedo and diameter,
we have access to the whole picture for only a few hundreds of asteroids. These
quantities are nevertheless very important to understand as they affect the
non-gravitational Yarkovsky effect responsible for meteorite delivery to Earth,
or the bulk composition and internal structure of asteroids.Comment: chapter that will appear in a Space Science Series book Asteroids I
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