5,855 research outputs found
A fast Bayesian approach to discrete object detection in astronomical datasets - PowellSnakes I
A new fast Bayesian approach is introduced for the detection of discrete
objects immersed in a diffuse background. This new method, called PowellSnakes,
speeds up traditional Bayesian techniques by: i) replacing the standard form of
the likelihood for the parameters characterizing the discrete objects by an
alternative exact form that is much quicker to evaluate; ii) using a
simultaneous multiple minimization code based on Powell's direction set
algorithm to locate rapidly the local maxima in the posterior; and iii)
deciding whether each located posterior peak corresponds to a real object by
performing a Bayesian model selection using an approximate evidence value based
on a local Gaussian approximation to the peak. The construction of this
Gaussian approximation also provides the covariance matrix of the uncertainties
in the derived parameter values for the object in question. This new approach
provides a speed up in performance by a factor of `hundreds' as compared to
existing Bayesian source extraction methods that use MCMC to explore the
parameter space, such as that presented by Hobson & McLachlan. We illustrate
the capabilities of the method by applying to some simplified toy models.
Furthermore PowellSnakes has the advantage of consistently defining the
threshold for acceptance/rejection based on priors which cannot be said of the
frequentist methods. We present here the first implementation of this technique
(Version-I). Further improvements to this implementation are currently under
investigation and will be published shortly. The application of the method to
realistic simulated Planck observations will be presented in a forthcoming
publication.Comment: 30 pages, 15 figures, revised version with minor changes, accepted
for publication in MNRA
A Wavelet-Based Algorithm for the Spatial Analysis of Poisson Data
Wavelets are scaleable, oscillatory functions that deviate from zero only
within a limited spatial regime and have average value zero. In addition to
their use as source characterizers, wavelet functions are rapidly gaining
currency within the source detection field. Wavelet-based source detection
involves the correlation of scaled wavelet functions with binned,
two-dimensional image data. If the chosen wavelet function exhibits the
property of vanishing moments, significantly non-zero correlation coefficients
will be observed only where there are high-order variations in the data; e.g.,
they will be observed in the vicinity of sources.
In this paper, we describe the mission-independent, wavelet-based source
detection algorithm WAVDETECT, part of the CIAO software package. Aspects of
our algorithm include: (1) the computation of local, exposure-corrected
normalized (i.e. flat-fielded) background maps; (2) the correction for exposure
variations within the field-of-view; (3) its applicability within the
low-counts regime, as it does not require a minimum number of background counts
per pixel for the accurate computation of source detection thresholds; (4) the
generation of a source list in a manner that does not depend upon a detailed
knowledge of the point spread function (PSF) shape; and (5) error analysis.
These features make our algorithm considerably more general than previous
methods developed for the analysis of X-ray image data, especially in the low
count regime. We demonstrate the algorithm's robustness by applying it to
various images.Comment: Accepted for publication in Ap. J. Supp. (v. 138 Jan. 2002). 61
pages, 23 figures, expands to 3.8 Mb. Abstract abridged for astro-ph
submissio
Near-infrared reddening of extra-galactic GMCs in a face-on geometry
[Abridged] We describe the near-infrared reddening signature of giant
molecular clouds (GMCs) in external galaxies. In particular, we examine the
E(J-H) and E(H-K) color-excesses, and the effective extinction law observed in
discrete GMC regions. We also study the effect of the relative scale height of
the GMC distribution to the color-excesses, and to the observed mass function
of GMCs. We perform Monte Carlo radiative transfer simulations with 3D models
of stellar radiation and clumpy dust distributions, resembling a face-on
geometry. The scattered light is included in the models, and near-infrared
color maps are calculated from the simulated data. The effective near-infrared
reddening law, i.e. the ratio E(J-H)/E(H-K), has a value close to unity in GMC
regions. The ratio depends on the relative scale height of GMCs, xi, and for xi
values 0.1...0.75 we find the typical ratios of 0.6...1.1. The effective
extinction law turns out to be very flat in GMC regions. We find the ratios of
apparent extinctions of A(H)/A(K)=1.35...1.55 and A(J)/A(H)=1.15. The effect of
the scattered flux on the effective reddening law, as well as on the effective
extinction law, is significant. Regarding the GMC mass function, we find no
correlation between the input and observed slopes of the mass functions.
Rather, the observed slope reflects the parameter and the dynamical range
of the mass function. We estimate that only a fraction of 10...20 % of the
total mass of GMCs is recovered, if the observed color-excess values are
transformed to masses using the Galactic reddening law. In the case of
individual clouds the fraction can vary between ~0...50 %.Comment: 8 pages, 10 figures, accepted for publication in A&A. Added missing
histograms in Fig.
Substructures in WINGS clusters
We search for and characterize substructures in the projected distribution of
galaxies observed in the wide field CCD images of the 77 nearby clusters of the
WIde-field Nearby Galaxy-cluster Survey (WINGS). This sample is complete in
X-ray flux in the redshift range 0.04<z<0.07. We search for substructures in
WINGS clusters with DEDICA, an adaptive-kernel procedure. We test the procedure
on Monte-Carlo simulations of the observed frames and determine the reliability
for the detected structures. DEDICA identifies at least one reliable structure
in the field of 55 clusters. 40 of these clusters have a total of 69
substructures at the same redshift of the cluster (redshift estimates of
substructures are from color-magnitude diagrams). The fraction of clusters with
subclusters (73%) is higher than in most studies. The presence of subclusters
affects the relative luminosities of the brightest cluster galaxies (BCGs).
Down to L ~ 10^11.2 L_Sun, our observed differential distribution of subcluster
luminosities is consistent with the theoretical prediction of the differential
mass function of substructures in cosmological simulations.Comment: A&A accepted - figure 6 is available from
http://adlibitum.oats.inaf.it/ramella/WINGSfig
Calculation of the detection properties in the binary symmetrical channel
summary:One of the important parts of railway signalling systems design is the safety of communication, achievable - among others - with the error detecting code. Getting evidence of quantitative safety targets, especially the probability of undetected error of the code, is a surprisingly complicated issue. We've analysed 2048 irreducible self-adjoint generator polynomials of the degree 32. More than 70 of these have a maximum probability of failure lower than the standard codes generally used. In this article we present the best of all codes we've analysed
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