33,075 research outputs found
Radio and optical interferometric imaging
Since diffraction-limited imaging with a single aperture yields angular resolution approx. lambda/D, the attainment of high angular resolution with single apertures requires the construction of correspondingly large monolithic apertures, the whole surface of which must be figured to much less than a wavelength. At the longer wavelengths, it is impossible to build a sufficiently large single aperture: for example, at lambda 21 cm, arcsec resolution requires an aperture of diameter approx. 50 km. At the shorter wavelengths, the atmosphere imposes a natural limit in resolution of about one arcsec. However, another route is possible; that is, using synthetic apertures to image the sky. Synthetic apertures are now in use in many fields, e.g., radio interferometry, radar imaging, and magnetic-resonance imaging. Radio-interferometric techniques developed in radio astronomy over the past 40 years are now being applied to optical and IR astronomical imaging by a number of groups. Furthermore, the problem of figuring synthetic apertures is considerably simpler, and can be implemented in a computer: new 'self-calibration' techniques allow imaging even in the presence of phase errors due to the atmosphere
The Effect of Unresolved Contaminant Stars on the Cross-Matching of Photometric Catalogues
A fundamental process in astrophysics is the matching of two photometric
catalogues. It is crucial that the correct objects be paired, and that their
photometry does not suffer from any spurious additional flux. We compare the
positions of sources in WISE, IPHAS, 2MASS, and APASS with Gaia DR1 astrometric
positions. We find that the separations are described by a combination of a
Gaussian distribution, wider than naively assumed based on their quoted
uncertainties, and a large wing, which some authors ascribe to proper motions.
We show that this is caused by flux contamination from blended stars not
treated separately. We provide linear fits between the quoted Gaussian
uncertainty and the core fit to the separation distributions.
We show that at least one in three of the stars in the faint half of a given
catalogue will suffer from flux contamination above the 1% level when the
density of catalogue objects per PSF area is above approximately 0.005. This
has important implications for the creation of composite catalogues. It is
important for any closest neighbour matches as there will be a given fraction
of matches that are flux contaminated, while some matches will be missed due to
significant astrometric perturbation by faint contaminants. In the case of
probability-based matching, this contamination affects the probability density
function of matches as a function of distance. This effect results in up to 50%
fewer counterparts being returned as matches, assuming Gaussian astrometric
uncertainties for WISE-Gaia matching in crowded Galactic plane regions,
compared with a closest neighbour match.Comment: 10 pages, 6 figures; accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Understanding the Heavy Tailed Dynamics in Human Behavior
The recent availability of electronic datasets containing large volumes of
communication data has made it possible to study human behavior on a larger
scale than ever before. From this, it has been discovered that across a diverse
range of data sets, the inter-event times between consecutive communication
events obey heavy tailed power law dynamics. Explaining this has proved
controversial, and two distinct hypotheses have emerged. The first holds that
these power laws are fundamental, and arise from the mechanisms such as
priority queuing that humans use to schedule tasks. The second holds that they
are a statistical artifact which only occur in aggregated data when features
such as circadian rhythms and burstiness are ignored. We use a large social
media data set to test these hypotheses, and find that although models that
incorporate circadian rhythms and burstiness do explain part of the observed
heavy tails, there is residual unexplained heavy tail behavior which suggests a
more fundamental cause. Based on this, we develop a new quantitative model of
human behavior which improves on existing approaches, and gives insight into
the mechanisms underlying human interactions.Comment: 9 pages in Physical Review E, 201
Algorithm 830: Another Visit With Standard and Modified Givens Transformations and A Remark on Algorithm 539
First we report on a correction and improvement to the Level 1 Blas routine srotmg for computing the Modified Givens Transformation (MG). We then, in the light of the performance of the code on modern compiler/hardware combinations, reconsider the strategy of supplying separate routines to compute and apply the transformation. Finally, we show that the apparent savings in multiplies obtained by using MG rather than the Standard Givens Transformation (SG) do not always translate into reductions in execution time
Modes of competition and the fitness of evolved populations
Competition between individuals drives the evolution of whole species.
Although the fittest individuals survive the longest and produce the most
offspring, in some circumstances the resulting species may not be optimally
fit. Here, using theoretical analysis and stochastic simulations of a simple
model ecology, we show how the mode of competition can profoundly affect the
fitness of evolved species. When individuals compete directly with one another,
the adaptive dynamics framework provides accurate predictions for the number
and distribution of species, which occupy positions of maximal fitness. By
contrast, if competition is mediated by the consumption of a common resource
then demographic noise leads to the stabilization of species with near minimal
fitness.Comment: 11 pages, 6 figure
Einstein supergravity amplitudes from twistor-string theory
This paper gives a twistor-string formulation for all tree amplitudes of
Einstein (super-)gravities for N=0 and 4. Formulae are given with and without
cosmological constant and with various possibilities for the gauging. The
formulae are justified by use of Maldacena's observation that conformal gravity
tree amplitudes with Einstein wave functions and non-zero cosmological constant
will correctly give the Einstein tree amplitudes. This justifies the
construction of Einstein gravity amplitudes at N=0 from twistor-string theory
and is extended to N=4 by requiring the standard relation between the
MHV-degree and the degree of the rational curve for Yang-Mills; this
systematically excludes the spurious conformal supergravity gravity
contributions. For comparison, BCFW recursion is used to obtain
twistor-string-like formulae at degree zero and one (anti-MHV and MHV) for
amplitudes with N=8 supersymmetry with and without cosmological constant.Comment: 20 pages. v2: minor corrections & clarification of relation to
formulae of Maldacena & Pimentel and Raju; v3: appendix on BCFW recursion
added, published version. v4: Full derivation for 3 point MHV amplitude now
include
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