117,453 research outputs found
Magnetic reconnection: flares and coronal heating in Active Galactic Nuclei
A magnetically-structured accretion disk corona, generated by buoyancy
instability in the disk, can account for observations of flare--like events in
Active Galactic Nuclei. We examine how Petschek magnetic reconnection,
associated with MHD turbulence, can result in a violent release of energy and
heat the magnetically closed regions of the corona up to canonical X-ray
emitting temperatures. X-ray magnetic flares, the after effect of the energy
released in slow shocks, can account for the bulk of the X-ray luminosity from
Seyfert galaxies and consistently explain the observed short-timescale
variability.Comment: revised version, 6 pages, 1 figures in MNRAS LaTex styl
Probing cosmological parameters with GRBs
In light of the recent finding of the narrow clustering of the
geometrically-corrected gamma-ray energies emitted by Gamma Ray Bursts (GRBs),
we investigate the possibility to use these sources as standard candles to
probe cosmological parameters such as the matter density Omega_m and the
cosmological constant energy density Omega_Lambda. By simulating different
samples of gamma-ray bursts, based on recent observational results, we find
that Omega_m (with the prior Omega_m + Omega_Lambda = 1) can be determined with
accuracy ~7% with data from 300 GRBs, provided a local calibration of the
standard candles be achieved.Comment: 4 pages, 2 figures, to appear in the Proceedings of the Conference
"30 Years of GRB Discovery", Santa Fe, New Mexico, USA, September 8-12, 200
Exchanges in complex networks: income and wealth distributions
We investigate the wealth evolution in a system of agents that exchange
wealth through a disordered network in presence of an additive stochastic
Gaussian noise. We show that the resulting wealth distribution is shaped by the
degree distribution of the underlying network and in particular we verify that
scale free networks generate distributions with power-law tails in the
high-income region. Numerical simulations of wealth exchanges performed on two
different kind of networks show the inner relation between the wealth
distribution and the network properties and confirm the agreement with a
self-consistent solution. We show that empirical data for the income
distribution in Australia are qualitatively well described by our theoretical
predictions.Comment: 8 pages, 11 figure
Vortices versus monopoles in color confinement
We construct the creation operator of a vortex for SU(2) pure gauge theory
using the methods developed for monopoles. We interpret its vacuum expectation
value as a disorder parameter for the deconfinement phase transition and find
that it behaves in the vacuum in a similar way to monopoles. Results are
extrapolated to the thermodynamical limit using finite-size scaling.Comment: Talk presented at Lattice2000 (Topology and Vacuum), 4 page
Semantic Matchmaking as Non-Monotonic Reasoning: A Description Logic Approach
Matchmaking arises when supply and demand meet in an electronic marketplace,
or when agents search for a web service to perform some task, or even when
recruiting agencies match curricula and job profiles. In such open
environments, the objective of a matchmaking process is to discover best
available offers to a given request. We address the problem of matchmaking from
a knowledge representation perspective, with a formalization based on
Description Logics. We devise Concept Abduction and Concept Contraction as
non-monotonic inferences in Description Logics suitable for modeling
matchmaking in a logical framework, and prove some related complexity results.
We also present reasonable algorithms for semantic matchmaking based on the
devised inferences, and prove that they obey to some commonsense properties.
Finally, we report on the implementation of the proposed matchmaking framework,
which has been used both as a mediator in e-marketplaces and for semantic web
services discovery
Confinement in QCD
The guiding lines of lattice investigations on colour confinement are
reviewed, together with recent results.Comment: 7 pages,3 figures, Plenary talk at PANIC'99,10-16 June 1999,Uppsal
The abelian projection revisited
Abelian projection is reanalysed in the frame of the SU(N) Higgs model. The
extension to QCD is discussed. It is shown that dual superconductivity of the
vacuum is an intrinsic property independent of the choice of the abelian
projection.Comment: 3 pages, 1 figure, Lattice2003(topology
Gamma-Ray Astronomy around 100 TeV with a large Muon Detector operated at Very High Altitude
Measurements at 100 TeV and above are an important goal for the next
generation of high energy gamma-ray astronomy experiments to solve the still
open problem of the origin of galactic cosmic rays. The most natural
experimental solution to detect very low radiation fluxes is provided by the
Extensive Air Shower (EAS) arrays. They benefit from a close to 90% duty cycle
and a very large field of view (about 2 sr), but the sensitivity is limited by
their angular resolution and their poor cosmic ray background discrimination.
Above 10 TeV the standard technique for rejecting the hadronic background
consists in looking for "muon-poor" showers.
In this paper we discuss the capability of a large muon detector (A=2500 m2)
operated with an EAS array at very high altitude (>4000 m a.s.l.) to detect
gamma-ray fluxes around 100 TeV. Simulation-based estimates of energy ranges
and sensitivities are presented.Comment: 4 pages, proceedings of the 30th ICRC, Merida, Mexico, 200
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