4,428 research outputs found
Optical/Multiwavelength Observations of GRB Afterglows
I review gamma-ray burst optical/multiwavelength afterglow observations since
1997, when the first counterparts to GRBs were discovered. I discuss what we
have learned from multiwavelength observations of GRB afterglows in relation to
the `standard' fireball plus relativistic blast-wave models. To first order the
`standard' model describes the afterglow observations well, but a wealth of
information can be gathered from the deviations of GRB afterglow observations
from this `standard' model. These deviations provide information on the nature
of the progenitor and on the physics of GRB production. In particular I focus
on the possible connection of GRBs to supernovae, on jet and circumstellar wind
models, on the early-time afterglow, and on the emission from the reverse
shock.Comment: 10 pages, 4 figures; invited review for the 5th Huntsville Gamma-Ray
Burst Workshop, eds. M. Kippen, R. Mallozzi, and G. Fishma
Discovery of a tight correlation between pulse lag/luminosity and jet-break times: a connection between gamma-ray burst and afterglow properties
A correlation is presented between the pulse lag and the jet-break time for
seven BATSE gamma-ray bursts with known redshifts. This is, to our best
knowledge, the first known direct tight correlation between a property of the
gamma-ray burst phase (the pulse lag) and the afterglow phase (the jet-break
time). As pulse lag and luminosity have been found to be correlated this also
represents a correlation between peak luminosity and jet-break time. Observed
timescales (variability or spectral lags) as well as peak luminosity naturally
have a strong dependence on the Lorentz factor of the outflow and so we propose
that much of the variety among GRBs has a purely kinematic origin (the speed or
direction of the outflow).
We explore a model in which the variation among GRBs is due to a variation in
jet-opening angles, and find that the narrowest jets have the fastest outflows.
We also explore models in which the jets have similar morphology and size, and
the variation among bursts is caused by variation in viewing angle and/or due
to a velocity profile. The relations between luminosity, variability, spectral
lag and jet-break time can be qualitatively understood from models in which the
Lorentz factor decreases as a function of angle from the jet axis. One expects
to see high luminosities, short pulse lags and high variability as well as an
early jet-break time for bursts viewed on axis, while higher viewing
inclinations will yield lower luminosities, longer pulse lags, smoother bursts
and later jet-break times.Comment: 10 pages, 3 figures, accepted to ApJ (new version contains minor
changes
Grossman’s Missing Health Threshold
We present a generalized solution to Grossman’s model of health capital (1972), relaxing the widely used assumption that individuals can adjust their health stock instantaneously to an “optimal” level without adjustment costs. The Grossman model then predicts the existence of a health threshold above which individuals do not demand medical care. Our generalized solution addresses a significant criticism: the model’s prediction that health and medical care are positively related is consistently rejected by the data. We suggest structural and reduced form equations to test our generalized solution and contrast the predictions of the model with the empirical literature.health, demand for health, health capital, medical care, labor
Efficiency analysis of reaction rate calculation methods using analytical models I: The 2D sharp barrier
We analyze the efficiency of different methods for the calculation of
reaction rates in the case of two simple analytical benchmark systems. Two
classes of methods are considered: the first are based on the free energy
calculation along a reaction coordinate and the calculation of the transmission
coefficient, the second on the sampling of dynamical pathways. We give scaling
rules for how this efficiency depends on barrier height and width, and we hand
out simple optimization rules for the method-specific parameters. We show that
the path sampling methods, using the transition interface sampling technique,
become exceedingly more efficient than the others when the reaction coordinate
is not the optimal one.Comment: 22 pages, 5 figure
Direct Transient Analysis of a Fuze Assembly by Axisymmetric Solid Elements
A fuze assembly, which consists of three major parts, nose, collar and sleeve, was designed to survive severe transverse impact giving a maximum base acceleration of 20.000 G. It is shown that hoop failure occurred in the collar after the impact. They also showed that by bonding the collar to the nose, the collar was able to survive the same impact. To find out the effectiveness of the bonding quantitatively, axisymmetric solid elements TRAPAX and TRIAAX were used in modelling the fuze and direct transient analysis was performed. The dynamic stresses in selected elements on the bonded and unbonded collars were compared. The peak hoop stresses in the unbonded collar were found to be up to three times higher than those in the bonded collar. The NASTRAN results explained the observed hoop failure in the unbonded collar. In addition, static and eigenvalue runs were performed as checks on the models prior to the transient runs. The use of the MPCAX cards and the existence and contributors of the calculated first several nearly identical natural frequencies are addressed
Feasibility investigation of a low-temperature, variable infrared source. Horizon definition study
Feasibility of low temperature, variable infrared source - calibration of instrumentation used in defining earth horizo
Asymptotic normalization of mirror states and the effect of couplings
Assuming that the ratio between asymptotic normalization coefficients of
mirror states is model independent, charge symmetry can be used to indirectly
extract astrophysically relevant proton capture reactions on proton-rich nuclei
based on information on stable isotopes. The assumption has been tested for
light nuclei within the microscopic cluster model. In this work we explore the
Hamiltonian independence of the ratio between asymptotic normalization
coefficients of mirror states when deformation and core excitation is
introduced in the system. For this purpose we consider a phenomenological rotor
+ N model where the valence nucleon is subject to a deformed mean field and the
core is allowed to excite. We apply the model to 8Li/8B, 13C/13N, 17O/17F,
23Ne/23Al, and 27Mg/27P. Our results show that for most studied cases, the
ratio between asymptotic normalization coefficients of mirror states is
independent of the strength and multipolarity of the couplings induced. The
exception is for cases in which there is an s-wave coupled to the ground state
of the core, the proton system is loosely bound, and the states have large
admixture with other configurations. We discuss the implications of our results
for novae.Comment: 8 pages, 2 figures, submitted to PR
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