2,852 research outputs found
A fireworks model for Gamma-Ray Bursts
The energetics of the long duration GRB phenomenon is compared with models of
a rotating Black Hole (BH) in a strong magnetic field generated by an accreting
torus. A rough estimate of the energy extracted from a rotating BH with the
Blandford-Znajek mechanism is obtained with a very simple assumption: an
inelastic collision between the rotating BH and the torus. The GRB energy
emission is attributed to an high magnetic field that breaks down the vacuum
around the BH and gives origin to a e+- fireball. Its subsequent evolution is
hypothesized, in analogy with the in-flight decay of an elementary particle, to
evolve in two distinct phases. The first one occurs close to the engine and is
responsible of energizing and collimating the shells. The second one consists
of a radiation dominated expansion, which correspondingly accelerates the
relativistic photon--particle fluid and ends at the transparency time. This
mechanism simply predicts that the observed Lorentz factor is determined by the
product of the Lorentz factor of the shell close to the engine and the Lorentz
factor derived by the expansion. An anisotropy in the fireball propagation is
thus naturally produced, whose degree depends on the bulk Lorentz factor at the
end of the collimation phase.Comment: Accepted for publication in MNRA
The GRB Variability/Peak Luminosity Correlation: new results
We report test results of the correlation between time variability and peak
luminosity of Gamma-Ray Bursts (GRBs), using a larger sample (32) of GRBs with
known redshift than that available to Reichart et al. (2001), and using as
variability measure that introduced by these authors. The results are puzzling.
Assuming an isotropic-equivalent peak luminosity, as done by Reichart et al.
(2001), a correlation is still found, but it is less relevant, and inconsistent
with a power law as previously reported. Assuming as peak luminosity that
corrected for GRB beaming for a subset of 16 GRBs with known beaming angle, the
correlation becomes little less significant.Comment: 11 pages, 10 figures, MNRAS, accepte
Intermittency and structure functions in channel flow turbulence
We present a study of intermittency in a turbulent channel flow. Scaling
exponents of longitudinal streamwise structure functions, ,
are used as quantitative indicators of intermittency.
We find that, near the center of the channel the values of
up to are consistent with the assumption of homogeneous/isotropic
turbulence. Moving towards the boundaries, we observe a growth of intermittency
which appears to be related to an intensified presence of ordered vortical
structures. In fact, the behaviour along the normal-to-wall direction of
suitably normalized scaling exponents shows a remarkable correlation with the
local strength of the Reynolds stress and with the \rms value of helicity
density fluctuations. We argue that the clear transition in the nature of
intermittency appearing in the region close to the wall, is related to a new
length scale which becomes the relevant one for scaling in high shear flows.Comment: 4 pages, 6 eps figure
Model and design of a double frequency piezoelectric resonator
A novel design of a multifrequency mechanical resonator with piezoelectric materials for energy harvesting is presented. The electromechanical response is described by a finite element model, which predicts the output voltage and the generated powe
A Class of String Backgrounds as a Semiclassical Limit of WZW Models
A class of string backgrounds associated with non semi-simple groups is
obtained as a special large level limit of ordinary WZW models. The models have
an integer Virasoro central charge and they include the background recently
studied by Nappi and Witten.Comment: 9 page
Eikonal Amplitude in the Gravireggeon Model at Superplanckian Energies
The gravity effects in high-energy scattering, described by a
four-dimensional eikonal amplitude related to gravireggeons induced by compact
extra dimensions are studied. It is demonstrated that the real part of the
eikonal (with a massless mode subtracted) dominates its imaginary part at both
small and large impact parameters, in contrast to the usual case of hadronic
high-energy behavior. The real part of the scattering amplitude exhibits an
exponential falloff at large momentum transfer, similar to that of the
imaginary part of the amplitude.Comment: 21 pages, LaTeX2e, no figure
A common stochastic process rules gamma-ray burst prompt emission and X-ray flares
Prompt gamma-ray and early X-ray afterglow emission in gamma-ray bursts
(GRBs) are characterized by a bursty behavior and are often interspersed with
long quiescent times. There is compelling evidence that X-ray flares are linked
to prompt gamma-rays. However, the physical mechanism that leads to the complex
temporal distribution of gamma-ray pulses and X-ray flares is not understood.
Here we show that the waiting time distribution (WTD) of pulses and flares
exhibits a power-law tail extending over 4 decades with index ~2 and can be the
manifestation of a common time-dependent Poisson process. This result is robust
and is obtained on different catalogs. Surprisingly, GRBs with many (>=8)
gamma-ray pulses are very unlikely to be accompanied by X-ray flares after the
end of the prompt emission (3.1 sigma Gaussian confidence). These results are
consistent with a simple interpretation: an hyperaccreting disk breaks up into
one or a few groups of fragments, each of which is independently accreted with
the same probability per unit time. Prompt gamma-rays and late X-ray flares are
nothing but different fragments being accreted at the beginning and at the end,
respectively, following the very same stochastic process and likely the same
mechanism.Comment: 11 pages, 7 figures, accepted by Ap
On the Zero-Slope Limit of the Compactified Closed Bosonic String
In the framework of the compactified closed bosonic string theory with the
extra spatial coordinates being circular with radius , we perform both the
zero-slope limit and the limit of the tree scattering
amplitude of four massless scalar particles. We explicitly show that this
double limit leads to amplitudes involving scalars which interact through the
exchange of a scalar, spin 1 and spin 2 particle. In particular, this latter
case reproduces the same result obtained in linearized quantum gravity.Comment: 10 pages, LaTex file, DSF-T-43/9
INSTANTON CALCULATIONS VERSUS EXACT RESULTS IN 4 DIMENSIONAL SUSY GAUGE THEORIES.
We relate the non-perturbative exact results in supersymmetry to perturbation
theory using several different methods: instanton calculations at weak or
strong coupling, a method using gaugino condensation and another method
relating strong and weak coupling. This allows many precise numerical checks of
the consistency of these methods, especially the amplitude of instanton
effects, and of the network of exact solutions in supersymmetry. However, there
remain difficulties with the instanton computations at strong coupling.Comment: 17 pages, uses harvmac
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