1,175 research outputs found
Hoisting frame facilitates handling of large objects
Hoisting frame can be used with a standard 5-ton forklift to handle the large spreader bars, or other bulky pieces of equipment, much faster and more efficiently than with a boom or gantry crane. In addition forklifts of this type are more readily available
Diversity Of Short Gamma-Ray Burst Afterglows From Compact Binary Mergers Hosting Pulsars
Short gamma-ray bursts (sGRBs) are widely believed to result from the mergers
of compact binaries. This model predicts an afterglow that bears the
characteristic signatures of a constant, low density medium, including a smooth
prompt-afterglow transition, and a simple temporal evolution. However, these
expectations are in conflict with observations for a non-negligible fraction of
sGRB afterglows. In particular, the onset of the afterglow phase for some of
these events appears to be delayed and, in addition, a few of them exhibit
late- time rapid fading in their lightcurves. We show that these peculiar
observations can be explained independently of ongoing central engine activity
if some sGRB progenitors are compact binaries hosting at least one pulsar. The
Poynting flux emanating from the pulsar companion can excavate a bow-shock
cavity surround- ing the binary. If this cavity is larger than the shock
deceleration length scale in the undisturbed interstellar medium, then the
onset of the afterglow will be delayed. Should the deceleration occur entirely
within the swept-up thin shell, a rapid fade in the lightcurve will ensue. We
identify two types of pulsar that can achieve the conditions necessary for
altering the afterglow: low field, long lived pulsars, and high field pulsars.
We find that a sizable fraction (~20-50%) of low field pulsars are likely to
reside in neutron star binaries based on observations, while their high field
counterparts are not. Hydrodynamical calculations motivated by this model are
shown to be in good agreement with observations of sGRB afterglow lightcurves.Comment: Accepted to ApjL. Direct comparison to observed X-Ray afterglows now
included. 5 Figure
Information retrieval system
Generalized information storage and retrieval system capable of generating and maintaining a file, gathering statistics, sorting output, and generating final reports for output is reviewed. File generation and file maintenance programs written for the system are general purpose routines
Search for Ferromagnetism in doped semiconductors in the absence of transition metal ions
In contrast to semiconductors doped with transition metal magnetic elements,
which become ferromagnetic at temperatures below ~ 100K, semiconductors doped
with non-magnetic ions (e.g. silicon doped with phosphorous) have not shown
evidence of ferromagnetism down to millikelvin temperatures. This is despite
the fact that for low densities the system is expected to be well modeled by
the Hubbard model, which is predicted to have a ferromagnetic ground state at
T=0 on 2- or 3-dimensional bipartite lattices in the limit of strong
correlation near half-filling. We examine the impurity band formed by
hydrogenic centers in semiconductors at low densities, and show that it is
described by a generalized Hubbard model which has, in addition to strong
electron-electron interaction and disorder, an intrinsic electron-hole
asymmetry. With the help of mean field methods as well as exact diagonalization
of clusters around half filling, we can establish the existence of a
ferromagnetic ground state, at least on the nanoscale, which is more robust
than that found in the standard Hubbard model. This ferromagnetism is most
clearly seen in a regime inaccessible to bulk systems, but attainable in
quantum dots and 2D heterostructures. We present extensive numerical results
for small systems that demonstrate the occurrence of high-spin ground states in
both periodic and positionally disordered 2D systems. We consider how
properties of real doped semiconductors, such as positional disorder and
electron-hole asymmetry, affect the ground state spin of small 2D systems. We
also discuss the relationship between this work and diluted magnetic
semiconductors, such as Ga_(1-x)Mn_(x)As, which though disordered, show
ferromagnetism at relatively high temperatures.Comment: 47 page
Cold Plasma Dispersion Relations in the Vicinity of a Schwarzschild Black Hole Horizon
We apply the ADM 3+1 formalism to derive the general relativistic
magnetohydrodynamic equations for cold plasma in spatially flat Schwarzschild
metric. Respective perturbed equations are linearized for non-magnetized and
magnetized plasmas both in non-rotating and rotating backgrounds. These are
then Fourier analyzed and the corresponding dispersion relations are obtained.
These relations are discussed for the existence of waves with positive angular
frequency in the region near the horizon. Our results support the fact that no
information can be extracted from the Schwarzschild black hole. It is concluded
that negative phase velocity propagates in the rotating background whether the
black hole is rotating or non-rotating.Comment: 27 pages, 11 figures accepted for publication in Gen. Relat. & Gravi
Embedded Sensors and Controls to Improve Component Performance and Reliability Conceptual Design Report
The objective of this project is to demonstrate improved reliability and increased performance made possible by deeply embedding instrumentation and controls (I&C) in nuclear power plant (NPP) components and systems. The project is employing a highly instrumented canned rotor, magnetic bearing, fluoride salt pump as its I&C technology demonstration platform. I&C is intimately part of the basic millisecond-by-millisecond functioning of the system; treating I&C as an integral part of the system design is innovative and will allow significant improvement in capabilities and performance. As systems become more complex and greater performance is required, traditional I&C design techniques become inadequate and more advanced I&C needs to be applied. New I&C techniques enable optimal and reliable performance and tolerance of noise and uncertainties in the system rather than merely monitoring quasistable performance. Traditionally, I&C has been incorporated in NPP components after the design is nearly complete; adequate performance was obtained through over-design. By incorporating I&C at the beginning of the design phase, the control system can provide superior performance and reliability and enable designs that are otherwise impossible. This report describes the progress and status of the project and provides a conceptual design overview for the platform to demonstrate the performance and reliability improvements enabled by advanced embedded I&C
Ferromagnetism in magnetically doped III-V semiconductors
The origin of ferromagnetism in semimagnetic III-V materials is discussed.
The indirect exchange interaction caused by virtual electron excitations from
magnetic impurity level in the bandgap to the valence band can explain
ferromagnetism in GaAs(Mn) no matter samples are degenerated or not. Formation
of ferromagnetic clusters and percolation picture of phase transition describes
well all available experimental data and allows to predict the Mn-composition
dependence of transition temperature in wurtzite (Ga,In,Al)N epitaxial layers.Comment: 4 pages with 3 figure
Wavelet and R/S analysis of the X-ray flickering of cataclysmic variables
Recently, wavelets and R/S analysis have been used as statistical tools to
characterize the optical flickering of cataclysmic variables. Here we present
the first comprehensive study of the statistical properties of X-ray flickering
of cataclysmic variables in order to link them with physical parameters. We
analyzed a sample of 97 X-ray light curves of 75 objects of all classes
observed with the XMM-Newton space telescope. By using the wavelets analysis,
each light curve has been characterized by two parameters, alpha and Sigma,
that describe the energy distribution of flickering on different timescales and
the strength at a given timescale, respectively. We also used the R/S analysis
to determine the Hurst exponent of each light curve and define their degree of
stochastic memory in time. The X-ray flickering is typically composed of long
time scale events (1.5 < alpha < 3), with very similar strengths in all the
subtypes of cataclysmic variables (-3 < Sigma < -1.5). The X-ray data are
distributed in a much smaller area of the alpha-Sigma parameter space with
respect to those obtained with optical light curves. The tendency of the
optical flickering in magnetic systems to show higher Sigma values than the
non-magnetic systems is not encountered in the X-rays. The Hurst exponents
estimated for all light curves of the sample are larger than those found in the
visible, with a peak at 0.82. In particular, we do not obtain values lower than
0.5. The X-ray flickering presents a persistent memory in time, which seems to
be stronger in objects containing magnetic white dwarf primaries. The
similarity of the X-ray flickering in objects of different classes together
with the predominance of a persistent stochastic behavior can be explained it
terms of magnetically-driven accretion processes acting in a considerable
fraction of the analyzed objects.Comment: 10 pages, 3 figures, 2 tables. Language revision. Accepted for
publication in A&
Transverse Wave Propagation in Relativistic Two-fluid Plasmas in de Sitter Space
We investigate transverse electromagnetic waves propagating in a plasma in
the de Sitter space. Using the 3+1 formalism we derive the relativistic
two-fluid equations to take account of the effects due to the horizon and
describe the set of simultaneous linear equations for the perturbations. We use
a local approximation to investigate the one-dimensional radial propagation of
Alfv\'en and high frequency electromagnetic waves and solve the dispersion
relation for these waves numerically.Comment: 19 pages, 12 figure
Persistent starspot signals on M dwarfs: multi-wavelength Doppler observations with the Habitable-zone Planet Finder and Keck/HIRES
Young, rapidly-rotating M dwarfs exhibit prominent starspots, which create
quasiperiodic signals in their photometric and Doppler spectroscopic
measurements. The periodic Doppler signals can mimic radial velocity (RV)
changes expected from orbiting exoplanets. Exoplanets can be distinguished from
activity-induced false positives by the chromaticity and long-term incoherence
of starspot signals, but these qualities are poorly constrained for
fully-convective M stars. Coherent photometric starspot signals on M dwarfs may
persist for hundreds of rotations, and the wavelength dependence of starspot RV
signals may not be consistent between stars due to differences in their
magnetic fields and active regions. We obtained precise multi-wavelength RVs of
four rapidly-rotating M dwarfs (AD Leo, G 227-22, GJ 1245B, GJ 3959) using the
near-infrared (NIR) Habitable-zone Planet Finder, and the optical Keck/HIRES
spectrometer. Our RVs are complemented by photometry from Kepler, TESS, and the
Las Cumbres Observatory (LCO) network of telescopes. We found that all four
stars exhibit large spot-induced Doppler signals at their rotation periods, and
investigated the longevity and optical-to-NIR chromaticity for these signals.
The phase curves remain coherent much longer than is typical for Sunlike stars.
Their chromaticity varies, and one star (GJ 3959) exhibits optical and NIR RV
modulation consistent in both phase and amplitude. In general, though, we find
that the NIR amplitudes are lower than their optical counterparts. We conclude
that starspot modulation for rapidly-rotating M stars frequently remains
coherent for hundreds of stellar rotations, and gives rise to Doppler signals
that, due to this coherence, may be mistaken for exoplanets.Comment: Accepted for publication in the Astrophysical Journa
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