11,497 research outputs found
A fast 2D image reconstruction algorithm from 1D data for the Gaia mission
A fast 2-dimensional image reconstruction method is presented, which takes as
input 1-dimensional data acquired from scans across a central source in
different orientations. The resultant reconstructed images do not show
artefacts due to non-uniform coverage in the orientations of the scans across
the central source, and are successful in avoiding a high background due to
contamination of the flux from the central source across the reconstructed
image. Due to the weighting scheme employed this method is also naturally
robust to hot pixels. This method was developed specifically with Gaia data in
mind, but should be useful in combining data with mismatched resolutions in
different directions.Comment: accepted (18 pages, 13 figures) will appear in Experimental Astronom
Conserving Approximations in Time-Dependent Density Functional Theory
In the present work we propose a theory for obtaining successively better
approximations to the linear response functions of time-dependent density or
current-density functional theory. The new technique is based on the
variational approach to many-body perturbation theory (MBPT) as developed
during the sixties and later expanded by us in the mid nineties. Due to this
feature the resulting response functions obey a large number of conservation
laws such as particle and momentum conservation and sum rules. The quality of
the obtained results is governed by the physical processes built in through
MBPT but also by the choice of variational expressions. We here present several
conserving response functions of different sophistication to be used in the
calculation of the optical response of solids and nano-scale systems.Comment: 11 pages, 4 figures, revised versio
From Hipparcos to Gaia
The measurement of the positions, distances, motions and luminosities of
stars represents the foundations of modern astronomical knowledge. Launched at
the end of the eighties, the ESA Hipparcos satellite was the first space
mission dedicated to such measurements. Hipparcos improved position accuracies
by a factor of 100 compared to typical ground-based results and provided
astrometric and photometric multi-epoch observations of 118,000 stars over the
entire sky. The impact of Hipparcos on astrophysics has been extremely valuable
and diverse. Building on this important European success, the ESA Gaia
cornerstone mission promises an even more impressive advance. Compared to
Hipparcos, it will bring a gain of a factor 50 to 100 in position accuracy and
of a factor of 10,000 in star number, collecting photometric,
spectrophotometric and spectroscopic data for one billion celestial objects.
During its 5-year flight, Gaia will measure objects repeatedly, up to a few
hundred times, providing an unprecedented database to study the variability of
all types of celestial objects. Gaia will bring outstanding contributions,
directly or indirectly, to most fields of research in astrophysics, such as the
study of our Galaxy and of its stellar constituents, the search for planets
outside the solar system.Comment: 6 pages. New Horizons in Time Domain Astronomy Proceedings IAU
Symposium No. 285, 2012, E. Griffin, B. Hanisch & R. Seaman, ed
On the Executability of Interactive Computation
The model of interactive Turing machines (ITMs) has been proposed to
characterise which stream translations are interactively computable; the model
of reactive Turing machines (RTMs) has been proposed to characterise which
behaviours are reactively executable. In this article we provide a comparison
of the two models. We show, on the one hand, that the behaviour exhibited by
ITMs is reactively executable, and, on the other hand, that the stream
translations naturally associated with RTMs are interactively computable. We
conclude from these results that the theory of reactive executability subsumes
the theory of interactive computability. Inspired by the existing model of ITMs
with advice, which provides a model of evolving computation, we also consider
RTMs with advice and we establish that a facility of advice considerably
upgrades the behavioural expressiveness of RTMs: every countable transition
system can be simulated by some RTM with advice up to a fine notion of
behavioural equivalence.Comment: 15 pages, 0 figure
Many-body Green's function theory for electron-phonon interactions: the Kadanoff-Baym approach to spectral properties of the Holstein dimer
We present a Kadanoff-Baym formalism to study time-dependent phenomena for
systems of interacting electrons and phonons in the framework of many-body
perturbation theory. The formalism takes correctly into account effects of the
initial preparation of an equilibrium state, and allows for an explicit
time-dependence of both the electronic and phononic degrees of freedom. The
method is applied to investigate the charge neutral and non-neutral excitation
spectra of a homogeneous, two-site, two-electron Holstein model. This is an
extension of a previous study of the ground state properties in the Hartree
(H), partially self-consistent Born (Gd) and fully self-consistent Born (GD)
approximations published in Ref. [arXiv:1403.2968]. We show that choosing a
homogeneous ground state solution leads to unstable dynamics for a sufficiently
strong interaction, and that allowing a symmetry-broken state prevents this.
The instability is caused by the bifurcation of the ground state and understood
physically to be connected with the bipolaronic crossover of the exact system.
This mean-field instability persists in the partially self-consistent Born
approximation but is not found for the fully self-consistent Born
approximation. By understanding the stability properties, we are able to study
the linear response regime by calculating the density-density response function
by time-propagation. This functions amounts to a solution of the Bethe-Salpeter
equation with a sophisticated kernel. The results indicate that none of the
approximations is able to describe the response function during or beyond the
bipolaronic crossover for the parameters investigated. Overall, we provide an
extensive discussion on when the approximations are valid, and how they fail to
describe the studied exact properties of the chosen model system.Comment: 12 figure
Wick Theorem for General Initial States
We present a compact and simplified proof of a generalized Wick theorem to
calculate the Green's function of bosonic and fermionic systems in an arbitrary
initial state. It is shown that the decomposition of the non-interacting
-particle Green's function is equivalent to solving a boundary problem for
the Martin-Schwinger hierarchy; for non-correlated initial states a one-line
proof of the standard Wick theorem is given. Our result leads to new
self-energy diagrams and an elegant relation with those of the imaginary-time
formalism is derived. The theorem is easy to use and can be combined with any
ground-state numerical technique to calculate time-dependent properties.Comment: 9 pages, 5 figure; extended version published in Phys. Rev.
Evaluation of three classifiers in mapping forest stand types using medium resolution imagery: a case study in the Offinso Forest District, Ghana
Loss of Ghana’s natural forests has been counteracted by plantation establishment and development. As at 2003, Ghana had a plantation area of about 97,000 hectares comprising different tree species. With the rapid expansion of plantations in the country, it is anticipated that major managerial challenges will arise due to insignificant technical personnel for monitoring and management. The application of GIS and remote sensing will be a powerful intervention and tool for monitoring and managing these forest resources in the area. The aim of this study was to determine a suitable method of mapping the different forest stand types using medium resolution images in the study area. Three classifiers were examined for their suitability in mapping the different forest stand types in the area (maximum likelihood, spectral angle mapper and decision tree). The results showed that using maximum likelihood classifier and ASTER imagery, different forest stand types can be accurately mapped with an overall accuracy of 88.50%.Key words: Geo-referenced, classification, algorithm, spectral, mapper
Land disputes and local conflict resolution mechanisms in Burundi
Over the year 2004, CED-CARITAS has been assisting the return of Burundian refugees and accompanied their reinsertion in their original communities. The progressive return of refugees accentuates the already existing pressure on agricultural land. Convinced that the question of land property is a key factor for sustainable peace, the Catholic Church of Burundi would like to start a project for ‘accompanying the peace process and reinsertion of victims in Burundi through the identification of land properties in dispute’. The first phase of the project consists of an identification and analysis of disputed land properties, to provide precise information on the nature and magnitude of the actually existing disputes about land. This research is meant to help decision takers in defining strategies for the prevention and peaceful resolution of disputes arising from the return of refugees. Hence, in cooperation with the Commission Episcopal Justice & Paix and its sub-offices in the communities, CED-Caritas has conducted a quantitative enquiry to identify all land problems and disputes existing in the different parishes of Burundi
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