349 research outputs found
Spectral Properties of Blast Wave Models of Gamma-Ray Burst Sources
We calculate the spectrum of blast wave models of gamma-ray burst sources,
for various assumptions about the magnetic field density and the relativistic
particle acceleration efficiency. For a range of physically plausible models we
find that the radiation efficiency is high, and leads to nonthermal spectra
with breaks at various energies comparable to those observed in the gamma-ray
range. Radiation is also predicted at other wavebands, in particular at X-ray,
optical/UV and GeV/TeV energies. We discuss the spectra as a function of
duration for three basic types of models, and for cosmological, halo and
galactic disk distances. We also evaluate the gamma-ray fluences and the
spectral characteristics for a range of external densities. Impulsive burst
models at cosmological distances can satisfy the conventional X-ray paucity
constraint S_x/S_\gamma \siml few percent over a wide range of durations, but
galactic models can do so only for bursts shorter than a few seconds, unless
additional assumptions are made. The emissivity is generally larger for bursts
in a denser external environment, with the efficiency increasing up to the
point where all the energy input is radiated away.Comment: 24 pages of Tex, plus 17 figures uuencoded tar-compressed postscript
file
A Power Flow Method for Radial Distribution Feeders with DER Penetration
This paper presents a novel power flow method suitable for radial distribution feeders, which consists a modification of the simplified power flow concept known as the DistFlow method, already available in the literature. The proposed method relies upon a differentiated manipulation of power losses, which are taken into account in voltage calculations, unlike other simplified methods, where losses are totally neglected. As a result, calculation accuracy is greatly improved, in terms of node voltages, losses and overall active & reactive power flows. In addition, the proposed method is non-iterative and entirely linear, being easily implementable and fast in execution. The method is particularly suited for feeders with a high penetration of Distributed Energy Resources (DER), providing results that closely match those of a full non-linear power flow and are considerably more accurate than the traditional linearized distribution power flow methods, without any increase in computational burden. The new method is applied to a variety of case studies in the paper, to demonstrate its accuracy and effectiveness, comparing its performance with the simplified (linearized) DistFlow and a conventional non-linear power flow method
Spectra of Unsteady Wind Models of Gamma-Ray Bursts
We calculate the spectra expected from unsteady relativistic wind models of
gamma-ray bursts, suitable for events of arbitrary duration. The spectral
energy distribution of the burst is calculated over photon energies spanning
from eV to TeV, for a range of event durations and variability timescales. The
relative strength of the emission at different wavelengths can provide valuable
information on the particle acceleration, radiation mechanisms and the possible
types of models.Comment: 10 pages, 2 postscript figures included, uses aaspp4.sty. Accepted
for publication in the Astrophysical Journal Letters. Also available at
http://www.astro.psu.edu/users/hara/Preprints/xxx_sub.p
Viscoelastic material behaviour of PBT-GF30 under thermo-mechanical cyclic loading
Abstract This paper deals with the simulation of the behaviour of a short glass fibre reinforced polybutylene terephthalate (PBT-GF30) under thermo-mechanical cyclic loading. Thermo-mechanical fatigue (TMF) tests, consisting of thermal cycling with a superimposed constant strain, have been carried out in the temperature ranges of -40 °C to 120 °C and -40 °C to 40 °C, applying different mean strain values. The main goal of the work is to model the stress trend during TMF cycles and assess the performance of a linear viscoelastic material model. A linear viscoelastic model has been implemented in ABAQUS 6.9-1 by means of Prony series, using the UTRS subroutine to model the time-temperature shift. The stress-time trend during TMF tests is discussed, comparing the simulated versus the experimental stress results. In particular, the maximum and minimum values within each cycle are considered, in order to evaluate the performance of the material model. Linear viscoelastic simulations show good agreement between experimental tests and FE analysis, both for plain and notched specimen
The presence of (NRPS) and (PKS) genes at the deepsea hydrothermal field in the Aegean Sea
Deep-sea hydrothermal vents are characterized by extremely high concentrations of microorganisms in stark contrast to the surrounding sea bottom. Nevertheless, deepsea consumers do not rapidly remove the high biomass of prey from these communities maybe due to vent microbes’ chemical defenses which still remain largely unknown. Meanwhile, the detection of genes responsible for antimicrobial and cytotoxic activity such as non-ribosomal peptide synthases (NRPS) and polyketide (PKS) of deep-sea vent bacteria has not so far been attempted
GRB 990123: Reverse and Internal Shock Flashes and Late Afterglow
The prompt (t \siml 0.16 days) light curve and initial 9-th magnitude
optical flash from GRB 990123 can be attributed to a reverse external shock, or
possibly to internal shocks. We discuss the time decay laws and spectral slopes
expected under various dynamical regimes, and discuss the constraints imposed
on the model by the observations, arguing that they provide strongly suggestive
evidence for features beyond those in the simple standard model. The longer
term afterglow behavior is discussed in the context of the forward shock, and
it is argued that, if the steepening after three days is due to a jet geometry,
this is likely to be due to jet-edge effects, rather than sideways expansion.Comment: M.N.R.A.S., subm. 2/26/99; (preprint uses aaspp4.sty), 9 page
Synchrotron Radiation as the Source of Gamma-Ray Burst Spectra
We investigate synchrotron emission models as the source of gamma-ray burst
spectra. We show that including the possibility for synchrotron
self-absorption, a ``smooth cutoff'' to the electron energy distribution, and
an anisotropic distribution for the electron pitch angles produces a whole
range of low energy spectral behavior. In addition, we show that the procedure
of spectral fitting to GRB data over a finite bandwidth can introduce a
spurious correlation between spectral parameters - in particular, the value of
the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index
alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be).
From this correlation and knowledge of the E_p distribution, we show how to
derive the expected distribution of alpha. We show that optically thin
synchrotron models with an isotropic electron pitch angle distribution can
explain the distribution of alpha below alpha=-2/3. This agreement is achieved
if we relax the unrealistic assumption of the presence of a sharp low energy
cutoff in the spectrum of accelerated electrons, and allow for a more gradual
break. We show that this low energy portion of the electron spectrum can be at
most flat. We also show that optically thin synchrotron models with an
anisotropic electron pitch angle distribution can explain all bursts with -2/3
< alpha <= 0$. The very few bursts with low energy spectral indices that fall
above alpha=0 may be due the presence of a the synchrotron self-absorption
frequency entering the lower end of the BATSE window. Our results also predict
a particular relationship between alpha and E_p during the temporal evolution
of a GRB. We give examples of spectral evolution in GRBs and discuss how the
behavior are consistent with the above models.Comment: 21 pages, including 10 postscript figures. To appear in the December
10, 2000 issue of Ap
Signs of low frequency dispersions in disordered binary dielectric mixtures (50-50)
Dielectric relaxation in disordered dielectric mixtures are presented by
emphasizing the interfacial polarization. The obtained results coincide with
and cause confusion with those of the low frequency dispersion behavior. The
considered systems are composed of two phases on two-dimensional square and
triangular topological networks. We use the finite element method to calculate
the effective dielectric permittivities of randomly generated structures. The
dielectric relaxation phenomena together with the dielectric permittivity
values at constant frequencies are investigated, and significant differences of
the square and triangular topologies are observed. The frequency dependent
properties of some of the generated structures are examined. We conclude that
the topological disorder may lead to the normal or anomalous low frequency
dispersion if the electrical properties of the phases are chosen properly, such
that for ``slightly'' {\em reciprocal mixture}--when , and
--normal, and while for ``extreme'' {\em reciprocal
mixture}--when , and --anomalous
low frequency dispersions are obtained. Finally, comparison with experimental
data indicates that one can obtain valuable information from simulations when
the material properties of the constituents are not available and of
importance.Comment: 13 pages, 7 figure
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