596 research outputs found
Efficiency and spectrum of internal gamma-ray burst shocks
We present an analysis of the Internal Shock Model of GRBs, where gamma-rays
are produced by internal shocks within a relativistic wind. We show that
observed GRB characteristics impose stringent constraints on wind and source
parameters. We find that a significant fraction, of order 20 %, of the wind
kinetic energy can be converted to radiation, provided the distribution of
Lorentz factors within the wind has a large variance and provided the minimum
Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s
is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy
breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time
R/c < 1 ms, suggesting a possible explanation for the observed clustering of
spectral break energies in this range. The lower limit to wind Lorenz factor
and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies
are set by Thomson optical depth due to electron positron pairs produced by
synchrotron photons. Natural consequences of the model are absence of bursts
with peak emission energy significantly exceeding 1 MeV, and existence of low
luminosity bursts with low, 1 keV to 10 keV, break energies.Comment: 10 pages, 5 ps-figures. Expanded discussion of magnetic field and
electron energy fraction. Accepted for publication in Astrophysical Journa
Comment on "Effects of spatial dispersion on electromagnetic surface modes and on modes associated with a gap between two half spaces"
Recently Bo E. Sernelius [Phys. Rev. B {\bf 71}, 235114 (2005)] investigated
the effects of spatial dispersion on the thermal Casimir force between two
metal half spaces. He claims that incorporating spatial dispersion results in a
negligible contribution from the transverse electric mode at zero frequency as
compared to the transverse magnetic mode. We demonstrate that this conclusion
is not reliable because, when applied to the Casimir effect, the approximate
description of spatial dispersion used is unjustified.Comment: 9 pages, minor corrections in accordance with the journal publication
have been mad
Dynamic Fluctuation Phenomena in Double Membrane Films
Dynamics of double membrane films is investigated in the long-wavelength
limit including the overdamped squeezing mode. We demonstrate that thermal
fluctuations essentially modify the character of the mode due to its nonlinear
coupling to the transversal shear hydrodynamic mode. The corresponding Green
function acquires as a function of the frequency a cut along the imaginary
semi-axis. Fluctuations lead to increasing the attenuation of the squeezing
mode it becomes larger than the `bare' value.Comment: 7 pages, Revte
Title Stabilization of Membrane Pores by Packing
We present a model for pore stabilization in membranes without surface
tension. Whereas an isolated pore is always unstable (since it either shrinks
tending to re-seal or grows without bound til to membrane disintegration), it
is shown that excluded volume interactions in a system of many pores can
stabilize individual pores of a given size in a certain range of model
parameters. For such a multipore membrane system, the distribution of pore size
and associated pore lifetime are calculated within the mean field
approximation. We predict that, above certain temperature when the effective
line tension becomes negative, the membrane exhibits a dynamic sieve-like
porous structure.Comment: 4 pages, 4 figure
Dynamics of nearly spherical vesicles in an external flow
We analytically derive an equation describing vesicle evolution in a fluid
where some stationary flow is excited regarding that the vesicle shape is close
to a sphere. A character of the evolution is governed by two dimensionless
parameters, and , depending on the vesicle excess area, viscosity
contrast, membrane viscosity, strength of the flow, bending module, and ratio
of the elongation and rotation components of the flow. We establish the ``phase
diagram'' of the system on the plane: we find curves corresponding
to the tank-treading to tumbling transition (described by the saddle-node
bifurcation) and to the tank-treading to trembling transition (described by the
Hopf bifurcation).Comment: 4 pages, 1 figur
Exploring the associations of culture with careers and the mediating role of HR practices:A conceptual model
Purpose:
The conceptual framework developed in the present study aims to highlight the importance of human resource (HR) practices as a mediator between national culture and employees' careers.
Design/methodology/approach:
The approach taken is a literature review and the development of a conceptual model.
Findings:
The paper contributes to the literature by focusing on how culture via HR practices might influence career success. Drawing on Hofstede's cultural dimensions, five propositions are developed regarding the impact of culture on careerârelevant HR practices, and how these practices are likely to influence employee career success.
Research limitations/implications:
Culture's effect should not be overstated. Looking at the propositions, it is possible that the influence of HR practices on career success is more pronounced than the direct effects of culture on career success. Future work is needed to measure and compare the relative strength of different associations as well as the possibility that other HR dimensions relevant to the study of career success may exist.
Originality/value:
At a general level, there is ample evidence of the impact of culture on the effectiveness of a variety of individual outcomes. The paper focused on the mediating role of HR practices as opposed to advancing hypotheses about direct relationships between culture and career success
Sound modes broadening for Fibonacci one dimensional quasicrystals
We investigate vibrational excitation broadening in one dimensional Fibonacci
model of quasicrystals (QCs). The chain is constructed from particles with two
masses following the Fibonacci inflation rule. The eigenmode spectrum depends
crucially on the mass ratio. We calculate the eigenstates and eigenfunctions.
All calculations performed self-consistently within the regular expansion over
the three wave coupling constant. The approach can be extended to three
dimensional systems. We find that in the intermediate range of mode coupling
constants, three-wave broadening for the both types of systems (1D Fibonacci
and 3D QCs) depends universally on frequency. Our general qualitative
conclusion is that for a system with a non-simple elementary cell phonon
spectrum broadening is always larger than for a system with a primitive cell
(provided all other characteristics are the same).Comment: 2o pages, 15 figure
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