3,318 research outputs found
On the theory of Gamma Ray Amplification through Stimulated Annihilation Radiation (GRASAR)
The theory of photon emission, absorption, and scattering in a relativistic plasma of positrons, electrons, and photon was studied. Expressions for the emissivities and absorption coefficients of pair annihilation, pair production, and Compton scattering are given and evaluated numerically. The conditions for negative absorption were investigated. In a system of photons and e(+) - e(-) pairs, an emission line at at approximately 0.43 MeV can be produced by grasar action provided that the pair chemical potential exceeds approximately 1 MeV. At a temperature of approximately 10 to the 9th power. This requires a pair density approximately 10 to the 30th power cm to the (-3) power a value much larger than the thermodynamic equilbrium pair density at this temperature. This emission line could account without a gravitational redshift for the observed lines at this energy from gamma ray bursts
Excavations at the Viking Barrow Cemetery at Heath Wood, Ingleby, Derbyshire
The cemetery at Heath Wood, Ingleby, Derbyshire, is the only known Scandinavian cremation cemetery in the British Isles. It comprises fifty-nine barrows, of which about one-third have been excavated on previous occasions, although earlier excavators concluded that some were empty cenotaph mounds. From 1998 to 2000 three barrows were examined. Our investigations have suggested that each of the barrows contained a burial, although not all contain evidence of a pyre. A full report of the 1998-2000 excavations is provided, alongside a summary of the earlier finds. The relationship of Heath Wood to the neighbouring site at Repton is examined, in order to understand its significance for the Scandinavian settlement of the Danelaw. It is concluded that Heath Wood may have been a war cemetery of the Viking Great Army of AD 873-8
Beltway: Getting Around Garbage Collection Gridlock
We present the design and implementation of a new garbage collection framework that significantly generalizes existing copying collectors. The Beltway framework exploits and separates object age and incrementality. It groups objects in one or more increments on queues called belts, collects belts independently, and collects increments on a belt in first-in-first-out order. We show that Beltway configurations, selected by command line options, act and perform the same as semi-space, generational, and older-first collectors, and encompass all previous copying collectors of which we are aware. The increasing reliance on garbage collected languages such as Java requires that the collector perform well. We show that the generality of Beltway enables us to design and implement new collectors that are robust to variations in heap size and improve total execution time over the best generational copying collectors of which we are aware by up to 40%, and on average by 5 to 10%, for small to moderate heap sizes. New garbage collection algorithms are rare, and yet we define not just one, but a new family of collectors that subsumes previous work. This generality enables us to explore a larger design space and build better collectors
Criterion for purely elastic Taylor-Couette instability in the flows of shear-banding fluids
In the past twenty years, shear-banding flows have been probed by various
techniques, such as rheometry, velocimetry and flow birefringence. In micellar
solutions, many of the data collected exhibit unexplained spatio-temporal
fluctuations. Recently, it has been suggested that those fluctuations originate
from a purely elastic instability of the flow. In cylindrical Couette geometry,
the instability is reminiscent of the Taylor-like instability observed in
viscoelastic polymer solutions. In this letter, we describe how the criterion
for purely elastic Taylor-Couette instability should be adapted to
shear-banding flows. We derive three categories of shear-banding flows with
curved streamlines, depending on their stability.Comment: 6 pages, 3 figure
Potential "ways of thinking" about the shear-banding phenomenon
Shear-banding is a curious but ubiquitous phenomenon occurring in soft
matter. The phenomenological similarities between the shear-banding transition
and phase transitions has pushed some researchers to adopt a 'thermodynamical'
approach, in opposition to the more classical 'mechanical' approach to fluid
flows. In this heuristic review, we describe why the apparent dichotomy between
those approaches has slowly faded away over the years. To support our
discussion, we give an overview of different interpretations of a single
equation, the diffusive Johnson-Segalman (dJS) equation, in the context of
shear-banding. We restrict ourselves to dJS, but we show that the equation can
be written in various equivalent forms usually associated with opposite
approaches. We first review briefly the origin of the dJS model and its initial
rheological interpretation in the context of shear-banding. Then we describe
the analogy between dJS and reaction-diffusion equations. In the case of
anisotropic diffusion, we show how the dJS governing equations for steady shear
flow are analogous to the equations of the dynamics of a particle in a quartic
potential. Going beyond the existing literature, we then draw on the Lagrangian
formalism to describe how the boundary conditions can have a key impact on the
banding state. Finally, we reinterpret the dJS equation again and we show that
a rigorous effective free energy can be constructed, in the spirit of early
thermodynamic interpretations or in terms of more recent approaches exploiting
the language of irreversible thermodynamics.Comment: 14 pages, 6 figures, tutorial revie
Nodular Lymphoid Hyperplasia in a Patient Initially Believed to Have Familial Adenomatous Polyposis
A 50-year-old male was initially thought to have familial adenomatous polyposis (FAP) after innumerable small nodules in the upper GI tract were discovered upon endoscopic retrograde cholangiopancreatography for common bile duct stone extraction. ERCP was unsuccessful due to inability to find the major papilla amongst the nodules found in the duodenum. Biopsy of the nodules was consistent with nodular lymphoid hyperplasia. The patient was later found to have common variable immunodeficiency
Social Identity Theory as a Framework for Understanding the Effects of Exposure to Positive Media Images of Self and Other on Intergroup Outcomes
Based on the tenets of social identity theory, a two-study experimental design was conducted which examined the influence of exposure to positive Latino media exemplars on ingroup (Latino) and outgroup (white) consumers. Specifically, the impact of exposure on interethnic evaluations as well as perceptions of self and other was evaluated. Results from Study 1 and Study 2 suggest that although exposure to positive media depictions of Latinos can prompt more favorable evaluations of ingroup and self among Latinos, the same cannot necessarily be said for the effects of exposure to affirmative messages on judgments about Latinos among outgroup members (i.e., whites). Instead, these data indicate that for whites, ingroup (racial/ethnic) identification is a potent factor in media-related interethnic evaluations. [ABSTRACT FROM AUTHOR
Development of the drag coefficient of a sphere translating through a viscoelastic fluid
Presentation at 15th OpenFOAM Workshop, June 22-25, 2020, Arlington, VA, USA
Thermal Comptonization in Mildly Relativistic Pair Plasmas
We use a Monte Carlo simulation to calculate the spectra of mildly
relativistic thermal plasmas in pair balance. We use the exact integral
expression for the electron-positron thermal annihilation spectrum, and provide
accurate expressions for the Gaunt factors of electron-ion, electron-electron,
and electron-positron thermal bremsstrahlung in the transrelativistic
temperature regime. The particles are assumed to be uniformly distributed
throughout a sphere, and the pair opacity is self-consistently calculated from
the energy and angular distribution of scattered photons. The resultant photon
spectra are compared with the nonrelativistic diffusion treatment of Sunyaev
and Titarchuk, the bridging formulas of Zdziarski, and the relativistic
corrections proposed by Titarchuk. We calculate allowed pair-balanced states of
thermal plasmas with no pair escape which include bremsstrahlung and internal
soft photons. The results are presented in the spectral index/compactness
plane, and can be directly compared with observations of spectra from AGNs and
Galactic black hole candidates. By comparing with X-ray spectral indices of
Seyfert AGNs and compactnesses inferred from X-ray variability data, we find
that the allowed solutions for pair equilibrium plasma imply that the
temperatures of Seyfert galaxies are keV. This prediction can be
tested with more sensitive gamma-ray observations of Seyfert galaxies. We find
that if the X-ray variability time scale gives an accurate measure of the
compactness, pair-dominated solutions are inconsistent with the data.Comment: 32 pages with 9 figures, compressed and uuencoded postscrip
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