3,482 research outputs found
Source extraction and photometry for the far-infrared and sub-millimeter continuum in the presence of complex backgrounds
(Abridged) We present a new method for detecting and measuring compact
sources in conditions of intense, and highly variable, fore/background. While
all most commonly used packages carry out the source detection over the signal
image, our proposed method builds from the measured image a "curvature" image
by double-differentiation in four different directions. In this way point-like
as well as resolved, yet relatively compact, objects are easily revealed while
the slower varying fore/background is greatly diminished. Candidate sources are
then identified by looking for pixels where the curvature exceeds, in absolute
terms, a given threshold; the methodology easily allows us to pinpoint
breakpoints in the source brightness profile and then derive reliable guesses
for the sources extent. Identified peaks are fit with 2D elliptical Gaussians
plus an underlying planar inclined plateau, with mild constraints on size and
orientation. Mutually contaminating sources are fit with multiple Gaussians
simultaneously using flexible constraints. We ran our method on simulated
large-scale fields with 1000 sources of different peak flux overlaid on a
realistic realization of diffuse background. We find detection rates in excess
of 90% for sources with peak fluxes above the 3-sigma signal noise limit; for
about 80% of the sources the recovered peak fluxes are within 30% of their
input values.Comment: Accepted on A&
Corruption and Positive Selection in Privatization
We consider the supply of a public good based on a publicly-owned facility. The Government has a choice between provision in-house and privatizing the facility and then outsourcing the production. In particular, we focus on corruption in the decision to privatize and on its effect on social welfare when there is asymmetric information on the public and private manager's efficiency. Our analysis shows that a corrupt Government, that chooses to privatize only in exchange for a bribe, makes a positive selection on the private firm's efficiency and, thus, may raise expected social welfare above what an honest Government could get.Corruption, Privatization, Private vs. public provision.
First principle theory of correlated transport through nano-junctions
We report the inclusion of electron-electron correlation in the calculation
of transport properties within an ab initio scheme. A key step is the
reformulation of Landauer's approach in terms of an effective transmittance for
the interacting electron system. We apply this framework to analyze the effect
of short range interactions on Pt atomic wires and discuss the coherent and
incoherent correction to the mean-field approach.Comment: 5 pages, 3 figure
A monolayer transition-metal dichalcogenide as a topological excitonic insulator
Monolayer transition-metal dichalcogenides in the T\u2032 phase could enable the realization of the quantum spin Hall effect1 at room temperature, because they exhibit a prominent spin\u2013orbit gap between inverted bands in the bulk2,3. Here we show that the binding energy of electron\u2013hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T\u2032 MoS2, which we investigate from first principles using many-body perturbation theory4. This paradoxical result hints at the instability of the T\u2032 phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed \u2018excitonic insulator\u2019 ground state5\u20137. Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers8\u201316 where the frustration between the two orders is relieved by breaking time reversal symmetry13,15,16. The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric17,18 as an effect of electron\u2013electron interaction
On the shape of the mass-function of dense clumps in the Hi-GAL fields. II. Using Bayesian inference to study the clump mass function
Context. Stars form in dense, dusty clumps of molecular clouds, but little is
known about their origin, their evolution and their detailed physical
properties. In particular, the relationship between the mass distribution of
these clumps (also known as the "clump mass function", or CMF) and the stellar
initial mass function (IMF), is still poorly understood. Aims. In order to
better understand how the CMF evolve toward the IMF, and to discern the "true"
shape of the CMF, large samples of bona-fide pre- and proto-stellar clumps are
required. Two such datasets obtained from the Herschel infrared GALactic Plane
Survey (Hi-GAL) have been described in paper I. Robust statistical methods are
needed in order to infer the parameters describing the models used to fit the
CMF, and to compare the competing models themselves. Methods. In this paper we
apply Bayesian inference to the analysis of the CMF of the two regions
discussed in Paper I. First, we determine the Bayesian posterior probability
distribution for each of the fitted parameters. Then, we carry out a
quantitative comparison of the models used to fit the CMF. Results. We have
compared the results from several methods implementing Bayesian inference, and
we have also analyzed the impact of the choice of priors and the influence of
various constraints on the statistical conclusions for the preferred values of
the parameters. We find that both parameter estimation and model comparison
depend on the choice of parameter priors. Conclusions. Our results confirm our
earlier conclusion that the CMFs of the two Hi-GAL regions studied here have
very similar shapes but different mass scales. Furthermore, the lognormal model
appears to better describe the CMF measured in the two Hi-GAL regions studied
here. However, this preliminary conclusion is dependent on the choice of
parameters priors.Comment: Submitted for publication to A&A on November 12, 2013. This paper
contains 11 pages and 7 figure
The BMW Deep X-ray Cluster Survey
We briefly describe the main features of the Brera Multi-Wavelet (BMW) survey
of serendipitous X-ray clusters, based on the still unexploited ROSAT-HRI
archival observations. Cluster candidates are selected from the general BMW
catalogue of 20,000 sources based exclusively on their X-ray extension.
Contrary to common wisdom, a clever selection of the HRI energy channels allows
us to significantly reduce the background noise, thus greatly improving the
ability to detect low surface-brightness sources as clusters. The resulting
sample of ~250 candidates shows a very good sky coverage down to a flux
\~3x10^-14 erg/s/cm^2 ([0.5-2.0] keV band), i.e comparable to existing
PSPC-based deep survey, with a particularly interesting area of ~100 sq.deg.
around fluxes ~10^-13 erg/s/cm^2, i.e. where highly-luminous, rare systems at
z~0.6-1 can be detected. At the same time, the superior angular resolution of
the instrument should avoid biases against intrinsically small systems, while
easing the identification process (e.g. by spotting blends and AGN
contaminants). While about 20% of the candidates are already identified with
groups/clusters at z<0.3 on the DSS2 images, we have started a deep CCD imaging
campaign to observe all sources associated to "blank fields". First results
from these observations reveal a distant (z>0.5) bonafide cluster counterpart
for ~80% of the targets.Comment: 3 pages, 2 figures; to appear in Proc. of the ESO/ECF/STSCI workshop
on "Deep Fields", Garching Oct 2000, (Publ: Springer
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