2,023 research outputs found
Probabilistic learning for selective dissemination of information
New methods and new systems are needed to filter or to selectively distribute the increasing volume of electronic information being produced nowadays. An effective information filtering system is one that provides the exact information that fulfills user's interests with the minimum effort by the user to describe it. Such a system will have to be adaptive to the user changing interest. In this paper we describe and evaluate a learning model for information filtering which is an adaptation of the generalized probabilistic model of information retrieval. The model is based on the concept of 'uncertainty sampling', a technique that allows for relevance feedback both on relevant and nonrelevant documents. The proposed learning model is the core of a prototype information filtering system called ProFile
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
Extremely energetic Fermi Gamma-Ray Bursts obey spectral energy correlations
The extremely energetic Fermi GRBs 080916C, with its Eiso of ~ 10^{55} erg in
1 keV - 10 GeV and intense GeV emission, and 090323 give us a unique
opportunity to test the reliability and extension of spectral energy
correlations. Based on Konus/WIND and Fermi spectral measurements, we find that
both events are fully consistent with the updated (95 events as of April 2009)
Ep,i - Eiso correlation, thus further confirming and extending it and pointing
against a possible flattening or increased dispersion at very high energies.
This also suggests that the physics behind the emission of peculiarly bright
and hard GRBs is the same as for softer and weaker ones. In addition, we find
that the normalization of the correlation obtained by considering these two
GRBs and the other long ones for which Ep,i was measured with high accuracy by
the Fermi/GBM are fully consistent with those obtained by other instruments
(e.g., BeppoSAX, Swift, Konus-WIND), thus indicating that the correlation is
not affected significantly by detectors limited thresholds and energy bands.
Prompted by the extension of the spectrum of GRB 080916C up to several GeVs
without any excess or cut-off, we also investigated if the evaluation of Eiso
in the commonly adopted 1 keV - 10 MeV energy band may bias the Ep,i - Eiso
correlation contributing to its scatter. By computing Eiso from 1 keV to 10
GeV, the slope of the correlation becomes slightly flatter, while its
dispersion does not change significantly. Finally, we find that GRB 080916C is
also consistent with most of the other spectral energy correlations derived
from it, with the possible exception of the Ep,i - Eiso - tb correlation.Comment: 9 pages, 5 figures, final revised version accepted for pubblication
in Astronomy & Astrophysics (main Journal
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
Interpretation of High Energy String Scattering in terms of String Configurations
High energy string scattering at fixed momentum transfer, known to be
dominated by Regge trajectory exchange, is interpreted by identifying families
of string states which induce each type of trajectory exchange. These include
the usual leading trajectory and its daughters as
well as the ``sister'' trajectories and their
daughters. The contribution of the sister to high energy scattering
is dominated by string excitations in the mode. Thus, at large ,
string scattering is dominated by wee partons, consistently with a picture of
string as an infinitely composite system of ``constituents'' which carry zero
energy and momentum.Comment: 14 pages, phyzzx, psfig required, Florida Preprint UFIFT-94-
Revisiting the diacylglycerol-induced insulin resistance hypothesis.
Obesity is associated with skeletal muscle insulin resistance, which is a crucial step in the development of type 2 diabetes. Among the mechanisms by which obesity may lead to insulin resistance, lipotoxicity is one of the hypotheses being explored; others include inflammation or the oxidative stress hypotheses. This review focuses on the role of diacylglycerols (DAG), a family of lipid metabolites implicated in the pathogenesis of lipotoxicity and insulin resistance. While recent studies report contradictory results in humans with regard to the importance of DAG-induced insulin resistance in skeletal muscle, other current literature highlight a potential role for DAG as signalling molecules. This review will discuss possible hypotheses explaining these contradictory results and the need to explore further the role of DAG in human metabolism
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
Intermittency and scaling laws for wall bounded turbulence
Well defined scaling laws clearly appear in wall bounded turbulence, even
very close to the wall, where a distinct violation of the refined Kolmogorov
similarity hypothesis (RKSH) occurs together with the simultaneous persistence
of scaling laws. A new form of RKSH for the wall region is here proposed in
terms of the structure functions of order two which, in physical terms,
confirms the prevailing role of the momentum transfer towards the wall in the
near wall dynamics.Comment: 10 pages, 5 figure
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