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