1,110 research outputs found
Stochastic geometry and topology of non-Gaussian fields
Gaussian random fields pervade all areas of science. However, it is often the
departures from Gaussianity that carry the crucial signature of the nonlinear
mechanisms at the heart of diverse phenomena, ranging from structure formation
in condensed matter and cosmology to biomedical imaging. The standard test of
non-Gaussianity is to measure higher order correlation functions. In the
present work, we take a different route. We show how geometric and topological
properties of Gaussian fields, such as the statistics of extrema, are modified
by the presence of a non-Gaussian perturbation. The resulting discrepancies
give an independent way to detect and quantify non-Gaussianities. In our
treatment, we consider both local and nonlocal mechanisms that generate
non-Gaussian fields, both statically and dynamically through nonlinear
diffusion.Comment: 8 pages, 4 figure
The Palermo Swift-BAT Hard X-ray Catalogue. II- Results after 39 months of sky survey
We present the Palermo Swift-BAT hard X-ray catalogue obtained from the
analysis of the the data relative to the first 39 months of the Swift mission.
We have developed a dedicated software to perform data reduction, mosaicking
and source detection on the BAT survey data. We analyzed the BAT dataset in
three energy bands (14-150 keV, 14-30 keV, 14-70 keV), obtaining a list of 962
detections above a significance threshold of 4.8 standard deviations. The
identification of the source counterparts was pursued using three strategies:
cross-correlation with published hard X-ray catalogues, analysis of field
observations of soft X-ray instruments, cross-correlation with the SIMBAD
databases. The survey covers 90% of the sky down to a flux limit of 2.5x10E-11
erg/cm2/s and 50% of the sky down to a flux limit of 1.8x10E-11 erg/cm2/s in
the 14-150 keV band. We derived a catalogue of 754 identified sources, of which
~69% are extragalactic, ~27% are Galactic objects, ~4% are already known X-ray
or gamma ray emitters whose nature has not been determined yet. The integrated
flux of the extragalactic sample is ~1% of the Cosmic X-ray background in the
14-150 keV range.Comment: Final version for pubblication in Astronomy and Astrophysics. 24
pages, 11 figures and 2 tables. The catalog is also available online at
http://bat.ifc.inaf.i
Direct ink writing of ultra-high temperature ceramics
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Luminosity-dependent evolution of soft X-ray selected AGN: New Chandra and XMM-Newton surveys
We present new results on the cosmological evolution of unabsorbed (type-1)
active galactic nuclei (AGN) selected in the soft (0.5-2 keV) X-ray band. From
a variety of ROSAT, XMM-Newton and Chandra surveys we selected a total of ~1000
AGN with an unprecedented spectroscopic and photometric optical/NIR
identification completeness. For the first time we are able to derive reliable
space densities for low-luminosity (Seyfert-type) X-ray sources at cosmological
redshifts. The evolutionary behaviour of AGN shows a strong dependence on X-ray
luminosity: while the space density of high-luminosity AGN reaches a peak
around z~2, similar to that of optically selected QSO, the space density of
low-luminosity AGNs peaks at redshifts below z=1. This confirms previous ROSAT
findings of a luminosity-dependent density evolution. Using a rigorous
treatment of the optical identification completeness we are able to show that
the space density of AGN with X-ray luminosities L_x < 10^45 erg s^-1 declines
significantly towards high redshifts.Comment: 21 pages, 13 figures, A&A (in press
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SYNTHESIS AND CHARACTERIZATION OF CARBON AEROGEL NANOCOMPOSITES CONTAINING DOUBLE-WALLED CARBON NANOTUBES
Carbon aerogels (CAs) are novel mesoporous materials with applications such as electrode materials for super capacitors and rechargeable batteries, adsorbents and advanced catalyst supports. To expand the potential application for these unique materials, recent efforts have focused on the design of CA composites with the goal of modifying the structure, conductivity or catalytic activity of the aerogel. Carbon nanotubes (CNTs) possess a number of intrinsic properties that make them promising materials in the design of composite materials. In addition, the large aspect ratios (100-1000) of CNTs means that small additions (less than 1 vol%) of CNTs can produce a composite with novel properties. Therefore, the homogeneous incorporation of CNTs into a CA matrix provides a viable route to new carbon-based composites with enhanced thermal, electrical and mechanical properties. One of the main challenges in preparing CNT composites is achieving a good uniform dispersion of nanotubes throughout the matrix. CAs are typically prepared through the sol-gel polymerization of resorcinol with formaldehyde in aqueous solution to produce organic gels that are supercritically dried and subsequently pyrolyzed in an inert atmosphere. Therefore, a significant issue in fabricating CA-CNT composites is dispersing the CNTs in the aqueous reaction media. Previous work in the design of CACNT composites have addressed this issue by using organic solvents in the sol-gel reaction to facilitate dispersion of the CNTs. To our knowledge, no data has been published involving the preparation of CA composites containing CNTs dispersed in aqueous media. In this report, we describe a new method for the synthesis of monolithic CA-CNT composites that involves the sol-gel polymerization of resorcinol and formaldehyde in an aqueous solution containing a surfactant-stabilized dispersion of double-walled carbon nanotubes (DWNT). One of the advantages of this approach is that it allows one to uniformly distribute CNTs in the CA matrix without compromising the synthetic control that is afforded by traditional organic sol-gel chemistry over the CA structure. We will describe the physical characterization of these novel materials as well as the influence of DWNT loading on the electrical conductivity of the CA composite
ELECTROPHORETIC DISPLAYS WITH TUNABLE, ANGLE-INDEPENDENT COLOR
Electrophoretic displays (EPDs), which exploit the surface charge of microparticles to control their deposition, have become widely available in consumer electronics, such as e-readers and smartwatches. However, a full-color EPD has yet to be demonstrated and commercialized. Here, we demonstrate colloidal assemblies of engineered quasi-amorphous photonic materials, using pigmentary α-Fe2O3/SiO2 core/shell nanoparticles, exhibiting non- iridescent tunable colors which can be tuned electrophoretically. The observed colors result from combination of colloidal particle arrangements, giving rise to structural color, along with the inherent pigmentary color of the α-Fe2O3/SiO2 nanoparticles. Colloidal particle assemblies of α-Fe2O3/SiO2 core/shell nanoparticles, and therefore the resulting colors, can be manipulated by shell thickness, particle concentration and external electrical stimuli. Dynamic tunability of α-Fe2O3/SiO2 nanomaterials in the visible wavelengths is demonstrated using reversible electrophoretic deposition with a noticeable difference between transmitted and reflected colors. The distinct contrast generated can be exploited for tunable display applications.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-704082
Data analysis of gravitational-wave signals from spinning neutron stars. III. Detection statistics and computational requirements
We develop the analytic and numerical tools for data analysis of the
gravitational-wave signals from spinning neutron stars for ground-based laser
interferometric detectors. We study in detail the statistical properties of the
optimum functional that need to be calculated in order to detect the
gravitational-wave signal from a spinning neutron star and estimate its
parameters. We derive formulae for false alarm and detection probabilities both
for the optimal and the suboptimal filters. We assess the computational
requirements needed to do the signal search. We compare a number of criteria to
build sufficiently accurate templates for our data analysis scheme. We verify
the validity of our concepts and formulae by means of the Monte Carlo
simulations. We present algorithms by which one can estimate the parameters of
the continuous signals accurately.Comment: LaTeX, 45 pages, 13 figures, submitted to Phys. Rev.
X-ray spectra of XMM-Newton serendipitous medium flux sources
We report on the results of a detailed analysis of the X-ray spectral
properties of a large sample of sources detected serendipitously with the
XMM-Newton observatory in 25 selected fields. The survey covers a total solid
angle of ~3.5 deg2 and contains 1137 sources with ~10E-15 < S0.5-10 < 10E-12
erg cm-2 s-1. We find evidence for hardening of the average X-ray spectra of
the sources towards fainter fluxes. We interpret this as indicating a higher
degree of photoelectric absorption amongst the fainter population. Absorption
is detected at 95% confidence in 20% of the sources, but it could certainly be
present in many other sources below our detection capabilities. For Broad Line
AGNs (BLAGNs), we detect absorption in ~10% of the sources with column
densities in the range 10E21 - 10E22 cm-2. The fraction of absorbed Narrow
Emission Line galaxies (NELGs, most with intrinsic X-ray luminosities >10E43
erg s-1, and therefore classified as type 2 AGNs) is significantly higher
(40%), with a hint of moderately higher columns. We do not find evidence for a
redshift evolution of the underlying power law index of BLAGNs, which stays
roughly constant at Gamma ~1.9, with intrinsic dispersion of 0.4. A small
fraction (~7%) of BLAGNs and NELGs require the presence of a soft excess, that
we model as a black body with temperature ranging from 0.1 to 0.3 keV.
Comparing our results on absorption to popular X-ray background synthesis
models, we find absorption in only ~40% of the sources expected. This is due to
a deficiency of heavily absorbed sources (with NH ~10E22 - 10E24 cm-2) in our
sample in comparison with the models. We therefore conclude that the synthesis
models require some revision in their specific parameters.Comment: 20 pages, 30 Postscript figures, A&A in pres
Tensor Regression with Applications in Neuroimaging Data Analysis
Classical regression methods treat covariates as a vector and estimate a
corresponding vector of regression coefficients. Modern applications in medical
imaging generate covariates of more complex form such as multidimensional
arrays (tensors). Traditional statistical and computational methods are proving
insufficient for analysis of these high-throughput data due to their ultrahigh
dimensionality as well as complex structure. In this article, we propose a new
family of tensor regression models that efficiently exploit the special
structure of tensor covariates. Under this framework, ultrahigh dimensionality
is reduced to a manageable level, resulting in efficient estimation and
prediction. A fast and highly scalable estimation algorithm is proposed for
maximum likelihood estimation and its associated asymptotic properties are
studied. Effectiveness of the new methods is demonstrated on both synthetic and
real MRI imaging data.Comment: 27 pages, 4 figure
Exciton Spin Dynamics in Semiconductor Quantum Wells
In this paper we will review Exciton Spin Dynamics in Semiconductor Quantum
Wells. The spin properties of excitons in nanostructures are determined by
their fine structure. We will mainly focus in this review on GaAs and InGaAs
quantum wells which are model systems.Comment: 55 pages, 27 figure
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