Modelling the Kinked Jet of the Crab Nebula

We investigate the dynamical propagation of the South-East jet from the Crab pulsar interacting with supernova ejecta by means of three-dimensional relativistic MHD numerical simulations with the PLUTO code. The initial jet structure is set up from the inner regions of the Crab Nebula. We study the evolution of hot, relativistic hollow outflows initially carrying a purely azimuthal magnetic field. Our jet models are characterized by different choices of the outflow magnetization ($\sigma$ parameter) and the bulk Lorentz factor ($\gamma_{j}$). We show that the jet is heavily affected by the growth of current-driven kink instabilities causing considerable deflection throughout its propagation length. This behavior is partially stabilized by the combined action of larger flow velocities and/or reduced magnetic field strengths. We find that our best jet models are characterized by relatively large values of $\sigma$ ($\gtrsim 1$) and small values of $\gamma_{j}\simeq 2$. Our results are in good agreement with the recent X-ray (\textit{Chandra}) data of the Crab Nebula South-East jet indicating that the jet changes direction of propagation on a time scale of the order of few years. The 3D models presented here may have important implications in the investigation of particle acceleration in relativistic outflows.Comment: 15 pages, 20 figure

MHD simulations of three-dimensional Resistive Reconnection in a cylindrical plasma column

Magnetic reconnection is a plasma phenomenon where a topological rearrangement of magnetic field lines with opposite polarity results in dissipation of magnetic energy into heat, kinetic energy and particle acceleration. Such a phenomenon is considered as an efficient mechanism for energy release in laboratory and astrophysical plasmas. An important question is how to make the process fast enough to account for observed explosive energy releases. The classical model for steady state magnetic reconnection predicts reconnection times scaling as $S^{1/2}$ (where $S$ is the Lundquist number) and yields times scales several order of magnitude larger than the observed ones. Earlier two-dimensional MHD simulations showed that for large Lundquist number the reconnection time becomes independent of $S$ ("fast reconnection" regime) due to the presence of the secondary tearing instability that takes place for $S \gtrsim 1 \times 10^4$. We report on our 3D MHD simulations of magnetic reconnection in a magnetically confined cylindrical plasma column under either a pressure balanced or a force-free equilibrium and compare the results with 2D simulations of a circular current sheet. We find that the 3D instabilities acting on these configurations result in a fragmentation of the initial current sheet in small filaments, leading to enhanced dissipation rate that becomes independent of the Lundquist number already at $S \simeq 1\times 10^3$.Comment: 11 pages, 11 figures, accepted for publication in MNRA

Sustainable Development as a Macro-Regional Goal

The idea of sustainable development emerged both from science and the environmental movement in the 70s and 80s of last century. Since then a lot was done in the field, sometimes without even naming it "sustainable development", but a lot still remains to be done. In this paper we start from the consideration that sustainable development requires that we see our world as a system that connects space and any discussion today must first consider the enormous impacts that modern globalization trends are playing in shaping the economic geography of places, such as macro-regions. In the future, macro-regional development could be the most important platform for enhancing sustainability. The regional dimension is suitable for redefining the meaning of sustainable social, economic and environmental systems in a way which is practical and very close to the people and to the operative public decisions (Alkan Olsson et al. 2004). This paper is divided in different sections: the first is about the regional sustainable development concept and place-based approach; the second is about Local Agenda 21; the third about regional integration and developmental regionalism; the fourth is about regional sustainable development in practise and the European macro-regions; at the end some conclusive remarks concerns the macro-regional future strategies addressing sustainable development

Flares in the Crab Nebula Driven by Untwisting Magnetic Fields

The recent discovery of PeV electrons from the Crab nebula, produced on rapid time scales of one day or less with a sharply peaked gamma-ray spectrum without hard X-rays, challenges traditional models of diffusive shock acceleration followed by synchrotron radiation. Here we outline an accleration model involving a DC electric field parallel to the magnetic field in a twisted toroidal field around the pulsar. Sudden developments of resistivity in localized regions of the twisted field are thought to drive the particle acceleration, up to PeV energies, resulting in flares. This model can reproduce the observed time scales of $T \approx 1$ day, the peak photon energies of $U_{\Phi,rr} \approx 1$ MeV, maximum electron energies of $U_{e,rr} \approx 1$ PeV, and luminosities of $L \approx 10^{36}$ erg s$^{-1}$.Comment: Astrophys. J. Letters, (accepted Apr 30, 2012; in press) - 1 figur

A Complementary Account to Emotion Extraction and Classification in Cultural Heritage Based on the Plutchik’s Theory

The paper presents a combined approach to knowledge-based emotion attribution and classification of cultural items employed in the H2020 project SPICE. In particular, we show a preliminary experimentation conducted on a selection of items contributed by the GAM Museum in Turin (Galleria di Arte Moderna), pointing out how different language-based approaches to emotion categorization (used in the systems Sophia and DEGARI respectively) can be powerfully combined to cope with both coverage and extended affective attributions. Interestingly, both approaches are based on an ontology of the Plutchik’s theory of emotions

Enumerating 3-generated axial algebras of Monster type

An axial algebra is a commutative non-associative algebra generated by axes, that is, primitive, semisimple idempotents whose eigenvectors multiply according to a certain fusion law. The Griess algebra, whose automorphism group is the Monster, is an example of an axial algebra. We say an axial algebra is of Monster type if it has the same fusion law as the Griess algebra. The $2$-generated axial algebras of Monster type, called Norton-Sakuma algebras, have been fully classified and are one of nine isomorphism types. In this paper, we enumerate and construct the $3$-generated axial algebras of Monster type which do not contain a $5\textrm{A}$, or $6\textrm{A}$ subalgebra.Comment: 27 pages. arXiv admin note: text overlap with arXiv:1804.0058