4,773 research outputs found
Measuring the transition to homogeneity with photometric redshift surveys
We study the possibility of detecting the transition to homogeneity using
photometric redshift catalogs. Our method is based on measuring the fractality
of the projected galaxy distribution, using angular distances, and relies only
on observable quantites. It thus provides a way to test the Cosmological
Principle in a model-independent unbiased way. We have tested our method on
different synthetic inhomogeneous catalogs, and shown that it is capable of
discriminating some fractal models with relatively large fractal dimensions, in
spite of the loss of information due to the radial projection. We have also
studied the influence of the redshift bin width, photometric redshift errors,
bias, non-linear clustering, and surveyed area, on the angular homogeneity
index H2 ({\theta}) in a {\Lambda}CDM cosmology. The level to which an upcoming
galaxy survey will be able to constrain the transition to homogeneity will
depend mainly on the total surveyed area and the compactness of the surveyed
region. In particular, a Dark Energy Survey (DES)-like survey should be able to
easily discriminate certain fractal models with fractal dimensions as large as
D2 = 2.95. We believe that this method will have relevant applications for
upcoming large photometric redshift surveys, such as DES or the Large Synoptic
Survey Telescope (LSST).Comment: 14 pages, 14 figure
A parametric approach for the identification of single-charged isotopes with AMS-02
Measurements of the isotopic composition of single-charged cosmic rays
provide important insights in the propagation processes. However, the isotopic
identification is challenging due to the one hundred times greater abundance of
protons when compared to deuterons, the only stable isotope of hydrogen. Taking
advantage of the precise measurements of the velocity and momentum in the Alpha
Magnetic Spectrometer (AMS-02), a particle physics detector operating aboard
the International Space Station since May 2011, we describe a parametric
template fit method, which takes into account systematic uncertainties such as
the fragmentation of particles inside AMS-02 and eventual differences between
data and simulation through the use of nuisance parameters. With this method we
are also able to assess the AMS-02 performance in terms of mass resolution,
showing that it is able to separate the isotopes of hydrogen up to 10 GeV/n.Comment: Accepted for publication in Nuclear Instruments and Methods in
Physics Research Section
Machine learning approach to the background reduction in singly charged cosmic-ray isotope measurements with AMS-02
Studying the isotopic composition of single-charge cosmic rays (CRs) provides essential data to investigate the CR propagation processes in our Galaxy. While current measurements are rare above 4 GeV/nucleon, the Alpha Magnetic Spectrometer (AMS-02) is able to measure the isotopic fluxes up to 10 GeV/n by combining the momentum measured by the silicon tracker with the precise measurements of the velocity provided by its Ring Imaging Cherenkov Detector (RICH). The correct measurement of the particles’ velocity is essential for identifying isotopes through their mass. This is particularly challenging for single-charge particles due to the low number of photons they produce in their Cherenkov rings, which makes the reconstruction easily disrupted by noise. Hence, identifying the sources and cleaning the sample from the background is essential for ensuring the quality of the rings. In this paper, we propose a novel approach to track the events whose mass is misidentified due to interactions inside the AMS-02 detector. Based on the actual location of these interactions, we propose a novel strategy to mitigate the background effectively and with high efficiency, which includes using cut-based selection criteria and a multivariate estimator based on the signals detected by the RICH.</p
Iterative-Bayesian unfolding of isotopic cosmic-ray fluxes measured by AMS-02
The measurement of the isotopic composition of cosmic rays (CRs) provides
essential insights the understanding of the origin and propagation of these
particles, namely the CR source spectra, the propagation processes and the
galactic halo size. The Alpha Magnetic Spectrometer (AMS-02), a CR detector
operating aboard the International Space Station since May 2011, has the
capability of performing these measurements due to its precise determination of
the velocity provided by its Time of Flight (TOF) and Ring Imaging Cherenkov
(RICH) detector. The correct interpretation of the data requires the
measurements to be deconvoluted from the instrumental effects. The unique
design of AMS-02, with more than one subdetector being used to measure the same
flux, requires a novel approach to unfold the measured fluxes. In this work, we
describe an iterative-Bayesian unfolding method applied in the context of
isotopic flux measurements in AMS-02. The accuracy of the method is assessed
using a simulated flux based on previous measurements and a full detector
response function. We introduce a non-parametric regularization method for the
detector response functions, as well as a single, smooth prior flux covering
the full range of measurements from both detectors, TOF and RICH. In addition,
the estimation of the errors and a discussion about the performance of the
method are also shown, demonstrating that the method is fast and reliable,
allowing for the recovery of the true fluxes in the full energy range.Comment: Submitted to NIM
A conjugate gradient method for the solution of the non-LTE line radiation transfer problem
This study concerns the fast and accurate solution of the line radiation
transfer problem, under non-LTE conditions. We propose and evaluate an
alternative iterative scheme to the classical ALI-Jacobi method, and to the
more recently proposed Gauss-Seidel and Successive Over-Relaxation (GS/SOR)
schemes. Our study is indeed based on the application of a preconditioned
bi-conjugate gradient method (BiCG-P). Standard tests, in 1D plane parallel
geometry and in the frame of the two-level atom model, with monochromatic
scattering, are discussed. Rates of convergence between the previously
mentioned iterative schemes are compared, as well as their respective timing
properties. The smoothing capability of the BiCG-P method is also demonstrated.Comment: Research note: 4 pages, 5 figures, accepted to A&
A Substantial Amount of Hidden Magnetic Energy in the Quiet Sun
Deciphering and understanding the small-scale magnetic activity of the quiet
solar photosphere should help to solve many of the key problems of solar and
stellar physics, such as the magnetic coupling to the outer atmosphere and the
coronal heating. At present, we can see only of the complex
magnetism of the quiet Sun, which highlights the need to develop a reliable way
to investigate the remaining 99%. Here we report three-dimensional radiative
tranfer modelling of scattering polarization in atomic and molecular lines that
indicates the presence of hidden, mixed-polarity fields on subresolution
scales. Combining this modelling with recent observational data we find a
ubiquitous tangled magnetic field with an average strength of G,
which is much stronger in the intergranular regions of solar surface convection
than in the granular regions. So the average magnetic energy density in the
quiet solar photosphere is at least two orders of magnitude greater than that
derived from simplistic one-dimensional investigations, and sufficient to
balance radiative energy losses from the solar chromosphere.Comment: 21 pages and 2 figures (letter published in Nature on July 15, 2004
Oviposição, desenvolvimento e reprodução de Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) em diferentes hospedeiros de importância econômica.
The host selection for oviposition by Spodoptera frugiperda (J.E. Smith) among corn, millet, cotton and soybean, and its relationship with the biological characteristics were investigated. Free and non-choice tests for oviposition using plots containing fi ve plants each, from each host in plastic greenhouse, resulted in similar oviposition preference among the host plants. In addition, selected biological characteristics of S. frugiperda were determined in the laboratory with larvae feeding on host leaves, and the combination of leaf and cotton boll. Neonate larvae exhibited low success of colonization on cotton boll compared to the leaves of all other hosts. Spodoptera frugiperda fed only on cotton bolls exhibited longer larval and pupal development, and longer adult life span; however with similar egg production. Larvae fed cotton leaves during six days and then transferred to cotton bolls, however, exhibited development and reproduction similar to those reared on corn or only on cotton leaves. Therefore, the variations on immature stages of S. frugiperda were not related with host selection for oviposition which was similar among the studied hosts. Based on our data, the millet as a winter, rotational, and cover crop is a potential host for S. frugiperda, while leaves and cotton bolls were diets of intermediate suitability as compared to corn and soybean leaves
Dichroic Masers due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization
The theory of the generation and transfer of polarized radiation, mainly
developed for interpreting solar spectropolarimetric observations, allows to
reconsider, in a more rigorous and elegant way, a physical mechanism that has
been suggested some years ago to interpret the high degree of polarization
often observed in astronomical masers. This mechanism, for which the name of
'dichroic maser' is proposed, can operate when a low density molecular cloud is
illuminated by an anisotropic source of radiation (like for instance a nearby
star). Here we investigate completely unsaturated masers and show that
selective stimulated emission processes are capable of producing highly
polarized maser radiation in a non-magnetic environment. The polarization of
the maser radiation is linear and is directed tangentially to a ring
equidistant to the central star. We show that the Hanle effect due to the
presence of a magnetic field can produce a rotation (from the tangential
direction) of the polarization by more that 45 degrees for some selected
combinations of the strength, inclination and azimuth of the magnetic field
vector. However, these very same conditions produce a drastic inhibition of the
maser effect. The rotations of about 90 degrees observed in SiO masers in the
evolved stars TX Cam by Kemball & Diamond (1997) and IRC+10011 by Desmurs et al
(2000) may then be explainedby a local modification of the anisotropy of the
radiation field, being transformed from mainly radial to mainly tangential.Comment: Accepted for publication on Ap
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