2,365 research outputs found
Continuous Local Symmetry in Ising-type Models
A class of generalized Ising models is examined with a view to extracting a
low energy sector comprising
Dirac fermions coupled to Yang-Mills vectors. The main feature of this
approach is a set of gap equations, covariant with respect to one of the
-dimensional crystallographic space groups.Comment: 7 pages, latex, IC/94/2
A cosmological independent calibration of the Ep,i-Eiso correlation for Gamma Ray Bursts
The relation connecting the emitted isotropic energy and the rest-frame peak
energy of the \nuF\nu spectra of Gamma-Ray Bursts (the Amati relation),
strictly depends on the cosmological model, so we need a method to obtain an
independent calibration of it. Using the Union Supernovae Ia catalog, we obtain
a cosmographic luminosity distance in the y-redshift and demonstrate that this
parametrization approximates very well the fiducial standard comsomlogical
model \LambdaCDM. Furthermore, by this cosmographic luminosity distance dl, it
is possible to achieve the Amati relation independent on the cosmological
model. The cosmographic Amati relation that we obtain agrees, in the errors,
with other cosmological-independent calibrations proposed in the literature.
This could be considered a good indication in view to obtain standard candles
by Gamma-Ray Bursts
Key words. Gamma rays : bursts - CosmologyComment: 6 pages, 3 figure
A search for pulsations in short gamma-ray bursts to constrain their progenitors
We searched for periodic and quasiperiodic signal in the prompt emission of a
sample of 44 bright short gamma-ray bursts detected with Fermi/GBM, Swift/BAT,
and CGRO/BATSE. The aim was to look for the observational signature of
quasiperiodic jet precession which is expected from black hole-neutron star
mergers, but not from double neutron star systems. Thus, this kind of search
holds the key to identify the progenitor systems of short GRBs and, in the wait
for gravitational wave detection, represents the only direct way to constrain
the progenitors. We tailored our search to the nature of the expected signal by
properly stretching the observed light curves by an increasing factor with
time, after calibrating the technique on synthetic curves. In none of the GRBs
of our sample we found evidence for periodic or quasiperiodic signals. In
particular, for the 7 unambiguously short GRBs with best S/N we obtained
significant upper limits to the amplitude of the possible oscillations. This
result suggests that BH-NS systems do not dominate the population of short GRB
progenitors as described by the kinematic model of Stone, Loeb, & Berger
(2013).Comment: 7 pages, 5 figures, accepted to ApJ, added reference
Average power density spectrum of long GRBs detected with BeppoSAX/GRBM and with Fermi/GBM
From past experiments the average power density spectrum (PDS) of GRBs with
unknown redshift was found to be modelled from 0.01 to 1 Hz with a power-law,
f^(-alpha), with alpha broadly consistent with 5/3. Recent analyses of the
Swift/BAT catalogue showed analogous results in the 15-150 keV band. We carried
out the same analysis on the bright GRBs detected by BeppoSAX/GRBM and
Fermi/GBM. The BeppoSAX/GRBM data, in the energy range 40-700 keV and with 7.8
and 0.5-ms time resolutions, allowed us to explore for the first time the
average PDS at very high frequencies (up to 1 kHz) and reveal a break around
1-2 Hz, previously found in CGRO/BATSE data. The Fermi/GBM data, in the energy
band 8-1000 keV, allowed us to explore for the first time the average PDS
within a broad energy range. Our results confirm and extend the energy
dependence of the PDS slope, according to which harder photons have shallower
PDS.Comment: 13 pages, 9 figures, accepted to MNRA
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 Induced Gravity in 2-d Topological Theories
We study 2-d gauge theories with the objective to understand, also
at the quantum level, the emergence of induced gravity. The wave functionals -
representing the eigenstates of a vanishing flat potential - are obtained in
the representation. The composition of the space they describe is then
analyzed: the state corresponding to the singlet representation of the gauge
group describes a topological universe. For other representations a metric
which is invariant under the residual gauge group is induced, apart from
possible topological obstructions. Being inherited from the group metric it is
rather rigid.Comment: 38, tex, 160/93/e
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
Classical Effective Field Theory for Weak Ultra Relativistic Scattering
Inspired by the problem of Planckian scattering we describe a classical
effective field theory for weak ultra relativistic scattering in which field
propagation is instantaneous and transverse and the particles' equations of
motion localize to the instant of passing. An analogy with the non-relativistic
(post-Newtonian) approximation is stressed. The small parameter is identified
and power counting rules are established. The theory is applied to reproduce
the leading scattering angle for either a scalar interaction field or
electro-magnetic or gravitational; to compute some subleading corrections,
including the interaction duration; and to allow for non-zero masses. For the
gravitational case we present an appropriate decomposition of the gravitational
field onto the transverse plane together with its whole non-linear action. On
the way we touch upon the relation with the eikonal approximation, some
evidence for censorship of quantum gravity, and an algebraic ring structure on
2d Minkowski spacetime.Comment: 29 pages, 2 figures. v4: Duration of interaction is determined in Sec
4 and detailed in App C. Version accepted for publication in JHE
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