71,968 research outputs found
High Energy Emission from the Starburst Galaxy NGC253
Measurement sensitivity in the energetic gamma-ray region has improved
considerably, and is about to increase further in the near future, motivating a
detailed calculation of high-energy (>100 MeV) and very-high-energy (VHE: >100
GeV) gamma-ray emission from the nearby starburst galaxy NGC253. Adopting the
convection-diffusion model for energetic electron and proton propagation, and
accounting for all the relevant hadronic and leptonic processes, we determine
the steady-state energy distributions of these particles by a detailed
numerical treatment. The electron distribution is directly normalized by the
measured synchrotron radio emission from the central starburst region; a
commonly expected theoretical relation is then used to normalize the proton
spectrum in this region. Doing so fully specifies the electron spectrum
throughout the galactic disk, and with an assumed spatial profile of the
magnetic field, the predicted radio emission from the full disk matches well
the observed spectrum, confirming the validity of our treatment. The resulting
radiative yields of both particles are calculated; the integrated HE and VHE
fluxes from the entire disk are predicted to be f(>100 MeV)~2x10^-8 cm^-2 s^-1
and f(>100 GeV)~4x10^-12 cm^-2 s^-1, respectively. We discuss the feasibility
of measuring emission at these levels with the space-borne Fermi and the
ground-based Cherenkov telescopes.Comment: 7 pages, 4 figures; accepted for publication in the MNRA
Comparison of Some Approaches to Determine Spatial Dependence of Soil Properties
Knowledge of variability and spatial structure of soil properties is essential for optimal design for collecting soil samples and effectively applying management decisions in the field. The objective of this study is to compare some approaches for characterizing, and comparing spatial dependence of isotropic second-order stationary processes. The evaluated approaches are the nugget to sill ratio (NR), normalized (by fitted sill) semivariogram, correlograms, and two integral scales. Soil samples, collected at a regular 50 m × 50 m grid from 0-15 cm depths, were analyzed for sand and clay, bulk density (b), saturated hydraulic conductivity (Ks), wilting point, available water content (AWC), pH, electrical conductivity (EC), nitrate-nitrogen (NO3- N), and chloride (Cl) were determined. Geostatistical software (GS+, Gamma Design Software, Plainwell, MI) was used to estimate the variance structure of various measured soil properties. Analysis include using data on the spatial variability of various properties from four published studies. NR displayed spatial dependence ignoring the influence of range, normalized semivariogram and correlogram provided the visual comparison, and both integral scales incorporated the influence of range and provided single number spatial dependence summaries. Either of the integral scale formulations can be used to characterize the spatial dependence of soil properties from agricultural fields
Location Dependent Dirichlet Processes
Dirichlet processes (DP) are widely applied in Bayesian nonparametric
modeling. However, in their basic form they do not directly integrate
dependency information among data arising from space and time. In this paper,
we propose location dependent Dirichlet processes (LDDP) which incorporate
nonparametric Gaussian processes in the DP modeling framework to model such
dependencies. We develop the LDDP in the context of mixture modeling, and
develop a mean field variational inference algorithm for this mixture model.
The effectiveness of the proposed modeling framework is shown on an image
segmentation task
A unifying representation for a class of dependent random measures
We present a general construction for dependent random measures based on
thinning Poisson processes on an augmented space. The framework is not
restricted to dependent versions of a specific nonparametric model, but can be
applied to all models that can be represented using completely random measures.
Several existing dependent random measures can be seen as specific cases of
this framework. Interesting properties of the resulting measures are derived
and the efficacy of the framework is demonstrated by constructing a
covariate-dependent latent feature model and topic model that obtain superior
predictive performance
Cell shape analysis of random tessellations based on Minkowski tensors
To which degree are shape indices of individual cells of a tessellation
characteristic for the stochastic process that generates them? Within the
context of stochastic geometry and the physics of disordered materials, this
corresponds to the question of relationships between different stochastic
models. In the context of image analysis of synthetic and biological materials,
this question is central to the problem of inferring information about
formation processes from spatial measurements of resulting random structures.
We address this question by a theory-based simulation study of shape indices
derived from Minkowski tensors for a variety of tessellation models. We focus
on the relationship between two indices: an isoperimetric ratio of the
empirical averages of cell volume and area and the cell elongation quantified
by eigenvalue ratios of interfacial Minkowski tensors. Simulation data for
these quantities, as well as for distributions thereof and for correlations of
cell shape and volume, are presented for Voronoi mosaics of the Poisson point
process, determinantal and permanental point processes, and Gibbs hard-core and
random sequential absorption processes as well as for Laguerre tessellations of
polydisperse spheres and STIT- and Poisson hyperplane tessellations. These data
are complemented by mechanically stable crystalline sphere and disordered
ellipsoid packings and area-minimising foam models. We find that shape indices
of individual cells are not sufficient to unambiguously identify the generating
process even amongst this limited set of processes. However, we identify
significant differences of the shape indices between many of these tessellation
models. Given a realization of a tessellation, these shape indices can narrow
the choice of possible generating processes, providing a powerful tool which
can be further strengthened by density-resolved volume-shape correlations.Comment: Chapter of the forthcoming book "Tensor Valuations and their
Applications in Stochastic Geometry and Imaging" in Lecture Notes in
Mathematics edited by Markus Kiderlen and Eva B. Vedel Jense
Modeling for seasonal marked point processes: An analysis of evolving hurricane occurrences
Seasonal point processes refer to stochastic models for random events which
are only observed in a given season. We develop nonparametric Bayesian
methodology to study the dynamic evolution of a seasonal marked point process
intensity. We assume the point process is a nonhomogeneous Poisson process and
propose a nonparametric mixture of beta densities to model dynamically evolving
temporal Poisson process intensities. Dependence structure is built through a
dependent Dirichlet process prior for the seasonally-varying mixing
distributions. We extend the nonparametric model to incorporate time-varying
marks, resulting in flexible inference for both the seasonal point process
intensity and for the conditional mark distribution. The motivating application
involves the analysis of hurricane landfalls with reported damages along the
U.S. Gulf and Atlantic coasts from 1900 to 2010. We focus on studying the
evolution of the intensity of the process of hurricane landfall occurrences,
and the respective maximum wind speed and associated damages. Our results
indicate an increase in the number of hurricane landfall occurrences and a
decrease in the median maximum wind speed at the peak of the season.
Introducing standardized damage as a mark, such that reported damages are
comparable both in time and space, we find that there is no significant rising
trend in hurricane damages over time.Comment: Published at http://dx.doi.org/10.1214/14-AOAS796 in the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
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