7,031 research outputs found
Some initial results and observations from a series of trials within the Ofcom TV White Spaces pilot
A Congeries of Absorption Cross Sections for Wavelengths Less Than 3000 degrees Angstrom
The absorption of ultraviolet solar radiation is of prime importance for the study of planetary atmospheres.The absorption coefficients of most of the atmospheric gases have been measured by a number of investigators, but the results are scattered throughout the literature. This report contains a detailed collection of absorption cross sections of the gases listed in Table 1 for wavelengths less than 3000 degrees angstroms. The data on each gas are given together with a historical sketch of the study of the gas and a list of the pertinent references. Also included is a study of the absorption and photoionization coefficients of the major atmospheric gases at intense solar emission lines
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I-Xe analysis of a magnetic separate from Lodranite GRA95209
I-Xe dating of a magnetic mineral separate from lodranite GRA95209 suggests that peak temperatures (and therefore melt migration) occurred early, at least a few million years before closure of the I-Xe system in phosphates from Acapulco
HST/STIS Imaging of the Host Galaxy of GRB980425/SN1998bw
We present HST/STIS observations of ESO 184-G82, the host galaxy of the
gamma-ray burst GRB 980425 associated with the peculiar Type Ic supernova
SN1998bw. ESO 184-G82 is found to be an actively star forming SBc sub-luminous
galaxy. We detect an object consistent with being a point source within the
astrometric uncertainty of 0.018 arcseconds of the position of the supernova.
The object is located inside a star-forming region and is at least one
magnitude brighter than expected for the supernova based on a simple
radioactive decay model. This implies either a significant flattening of the
light curve or a contribution from an underlying star cluster.Comment: 12 pages, 2 figures, AASTeX v5.02 accepted for publication in ApJ
Letter
Parasites, Porotic Hyperostosis, and the Implications of Changing Perspectives
Cribra orbitalia and porotic hyperostosis traditionally have been viewed (at least by archaeologists) as indicators of chronic iron deficiency anemia resulting from a dependency upon maize. Recent interpretations, however, have sought to explain these conditions as an evolutionary, adaptive response to intestinal parasites rather than as a consequence of poor nutrition. Thus diet is eliminated as a contributing factor. This model, however, adopts too simplistic a view of evolution. Furthermore, it concomitantly severs the well-documented link that exists between cranial lesions and cereal-based subsistence. A more realistic approach would be to incorporate both diet and pathogens (bacterial as well as parasitic) into a symbiotic model that acknowledges the important role of parasites in the etiology of cribra orbitalia and porotic hyperostosis, while maintaining the diagnostic value of these conditions as hallmarks of early agriculture
The Role of Adaptation in Archaeological Explanation
Adaptation, a venerable icon in archaeology, often is afforded the vacuous role of being an ex-post-facto argument used to »explain» the appearance and persistence of traits among prehistoric groups- A position that has seriously impeded development of a selectionist perspective in archaeology. Biological and philosophical definitions of adaptation- A nd by extension, definitions of adaptedness-vary considerably, but all are far removed from those usually employed in archaeology. The prevailing view in biology is that adaptations are features that were shaped by natural selection and that increase the adaptedness of an organism. Thus adaptations are separated from other features that may contribute to adaptedness but are products of other evolutionary processes. Analysis of adaptation comprises two stages: Showing that a feature was under selection and how the feature functioned relative to the potential adaptedness of its bearers. The archaeological record contains a wealth of information pertinent to examining the adaptedness of prehistoric groups, but attempts to use it will prove successful only if a clear understanding exists of what adaptation is and is not
Quantum Kinetic Theory III: Simulation of the Quantum Boltzmann Master Equation
We present results of simulations of a em quantum Boltzmann master equation
(QBME) describing the kinetics of a dilute Bose gas confined in a trapping
potential in the regime of Bose condensation. The QBME is the simplest version
of a quantum kinetic master equations derived in previous work. We consider two
cases of trapping potentials: a 3D square well potential with periodic boundary
conditions, and an isotropic harmonic oscillator. We discuss the stationary
solutions and relaxation to equilibrium. In particular, we calculate particle
distribution functions, fluctuations in the occupation numbers, the time
between collisions, and the mean occupation numbers of the one-particle states
in the regime of onset of Bose condensation.Comment: 12 pages, 15 figure
Critical phenomena in exponential random graphs
The exponential family of random graphs is one of the most promising class of
network models. Dependence between the random edges is defined through certain
finite subgraphs, analogous to the use of potential energy to provide
dependence between particle states in a grand canonical ensemble of statistical
physics. By adjusting the specific values of these subgraph densities, one can
analyze the influence of various local features on the global structure of the
network. Loosely put, a phase transition occurs when a singularity arises in
the limiting free energy density, as it is the generating function for the
limiting expectations of all thermodynamic observables. We derive the full
phase diagram for a large family of 3-parameter exponential random graph models
with attraction and show that they all consist of a first order surface phase
transition bordered by a second order critical curve.Comment: 14 pages, 8 figure
A Heterosynaptic Learning Rule for Neural Networks
In this article we intoduce a novel stochastic Hebb-like learning rule for
neural networks that is neurobiologically motivated. This learning rule
combines features of unsupervised (Hebbian) and supervised (reinforcement)
learning and is stochastic with respect to the selection of the time points
when a synapse is modified. Moreover, the learning rule does not only affect
the synapse between pre- and postsynaptic neuron, which is called homosynaptic
plasticity, but effects also further remote synapses of the pre- and
postsynaptic neuron. This more complex form of synaptic plasticity has recently
come under investigations in neurobiology and is called heterosynaptic
plasticity. We demonstrate that this learning rule is useful in training neural
networks by learning parity functions including the exclusive-or (XOR) mapping
in a multilayer feed-forward network. We find, that our stochastic learning
rule works well, even in the presence of noise. Importantly, the mean learning
time increases with the number of patterns to be learned polynomially,
indicating efficient learning.Comment: 19 page
The Host Galaxy and Optical Light Curve of the Gamma-Ray Burst GRB 980703
We present deep HST/STIS and ground-based photometry of the host galaxy of
the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the
burst. We find that the host is a blue, slightly over-luminous galaxy with
V_gal = 23.00 +/- 0.10, (V-R)_gal = 0.43 +/- 0.13, and a centre that is
approximately 0.2 mag bluer than the outer regions of the galaxy. The galaxy
has a star-formation rate of 8-13 M_sun/yr, assuming no extinction in the host.
We find that the galaxy is best fit by a Sersic R^(1/n) profile with n ~= 1.0
and a half-light radius of 0.13 arcsec (= 0.72/h_100 proper kpc). This
corresponds to an exponential disk with a scale radius of 0.22 arcsec (=
1.21/h_100 proper kpc). Subtracting a fit with elliptical isophotes leaves
large residuals, which suggests that the host galaxy has a somewhat irregular
morphology, but we are unable to connect the location of GRB 980703 with any
special features in the host. The host galaxy appears to be a typical example
of a compact star forming galaxy similar to those found in the Hubble Deep
Field North. The R-band light curve of the optical afterglow associated with
this gamma-ray burst is consistent with a single power-law decay having a slope
of alpha = -1.37 +/- 0.14. Due to the bright underlying host galaxy the late
time properties of the light-curve are very poorly constrained. The decay of
the optical light curve is consistent with a contribution from an underlying
Type Ic supernova like SN1998bw, or a dust echo, but such contributions cannot
be securely established.Comment: 9 pages, 5 figures, LaTeX using A&A Document Class v4.05, to appear
in A&
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