13,720 research outputs found
Potential solar axion signatures in X-ray observations with the XMM-Newton observatory
The soft X-ray flux produced by solar axions in the Earth's magnetic field is
evaluated in the context of ESA's XMM-Newton observatory. Recent calculations
of the scattering of axion-conversion X-rays suggest that the sunward
magnetosphere could be an observable source of 0.2-10 keV photons. For
XMM-Newton, any conversion X-ray intensity will be seasonally modulated by
virtue of the changing visibility of the sunward magnetic field region. A
simple model of the geomagnetic field is combined with the ephemeris of
XMM-Newton to predict the seasonal variation of the conversion X-ray intensity.
This model is compared with stacked XMM-Newton blank sky datasets from which
point sources have been systematically removed. Remarkably, a seasonally
varying X-ray background signal is observed. The EPIC count rates are in the
ratio of their X-ray grasps, indicating a non-instrumental, external photon
origin, with significances of 11(pn), 4(MOS1) and 5(MOS2) sigma. After
examining the constituent observations spatially, temporally and in terms of
the cosmic X-ray background, we conclude that this variable signal is
consistent with the conversion of solar axions in the Earth's magnetic field.
The spectrum is consistent with a solar axion spectrum dominated by
bremsstrahlung- and Compton-like processes, i.e. axion-electron coupling
dominates over axion-photon coupling and the peak of the axion spectrum is
below 1 keV. A value of 2.2e-22 /GeV is derived for the product of the
axion-photon and axion-electron coupling constants, for an axion mass in the
micro-eV range. Comparisons with limits derived from white dwarf cooling may
not be applicable, as these refer to axions in the 0.01 eV range. Preliminary
results are given of a search for axion-conversion X-ray lines, in particular
the predicted features due to silicon, sulphur and iron in the solar core, and
the 14.4 keV transition line from 57Fe.Comment: Accepted for publication in MNRAS. 67 pages total, including 39
figures, 6 table
It\u27s A Grand Old Flag To Fight For
https://digitalcommons.library.umaine.edu/mmb-vp/2896/thumbnail.jp
A critical role for Cadherin6B in regulating avian neural crest emigration
Neural crest cells originate in the dorsal neural tube but subsequently undergo an epithelial-to-mesenchymal transition (EMT), delaminate, and migrate to diverse locations in the embryo where they contribute to a variety of derivatives. Cadherins are a family of cell–cell adhesion molecules expressed in a broad range of embryonic tissues, including the neural tube. In particular, cadherin6B (Cad6B) is expressed in the dorsal neural tube prior to neural crest emigration but is then repressed by the transcription factor Snail2, expressed by premigratory and early migrating cranial neural crest cells. To examine the role of Cad6B during neural crest EMT, we have perturbed Cad6B protein levels in the cranial neural crest-forming region and have examined subsequent effects on emigration and migration. The results show that knock-down of Cad6B leads to premature neural crest cell emigration, whereas Cad6B overexpression disrupts migration. Our data reveal a novel role for Cad6B in controlling the proper timing of neural crest emigration and delamination from the neural tube of the avian embryo
Sequential category aggregation and partitioning approaches for multi-way contingency tables based on survey and census data
Large contingency tables arise in many contexts but especially in the
collection of survey and census data by government statistical agencies.
Because the vast majority of the variables in this context have a large number
of categories, agencies and users need a systematic way of constructing tables
which are summaries of such contingency tables. We propose such an approach in
this paper by finding members of a class of restricted log-linear models which
maximize the likelihood of the data and use this to find a parsimonious means
of representing the table. In contrast with more standard approaches for model
search in hierarchical log-linear models (HLLM), our procedure systematically
reduces the number of categories of the variables. Through a series of
examples, we illustrate the extent to which it can preserve the interaction
structure found with HLLMs and be used as a data simplification procedure prior
to HLL modeling. A feature of the procedure is that it can easily be applied to
many tables with millions of cells, providing a new way of summarizing large
data sets in many disciplines. The focus is on information and description
rather than statistical testing. The procedure may treat each variable in the
table in different ways, preserving full detail, treating it as fully nominal,
or preserving ordinality.Comment: Published in at http://dx.doi.org/10.1214/08-AOAS175 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Effect on Sward Botanical Composition of Mixed and Sequential Grazing by Cattle and Sheep of Upland Permanent Pasture in the UK
Previous work has shown benefits of sequential grazing by cattle and sheep, with superior liveweight gains being recorded for lambs grazing swards previously grazed by cattle. A preliminary study of the effects of mixed sheep and cattle grazing also suggested that more extensively grazed swards offer scope for complementary grazing between sheep and cattle. The aim of this experiment was to directly compare these two approaches of integrating the grazing of cattle and sheep
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