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In this review we describe recent observational and theoretical developments in our understanding of pulsar winds and pulsar-wind nebulae (PWNe). We put special emphasis on the results from observations of well-characterized PWNe of various types (e.g., torus-jet and bowshock-tail), the most recent MHD modeling efforts, and the status of the flaring Crab PWN puzzle., Comment: 60 pages, 20 figures; slightly modified version of the article published in Space Science Reviews; citations added in the revised version

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 $S \simeq 1\times 10^3$., 11 pages, 11 figures, accepted for publication in MNRAS

The ARGO-YBJ air shower detector monitored the Crab Nebula gamma ray emission from 2007 November to 2013 February. The integrated signal, consisting of $\sim$3.3 $\times$ 10$^5$ events,reached the statistical significance of 21.1 standard deviations. The obtained energy spectrum in the energy range 0.3-20 TeV can be described by a power law function dN/dE = I$_0$ (E / 2 TeV)$^{-\alpha}$, with a flux normalization I$_0$ = (5.2 $\pm$ 0.2) $\times$ 10$^{-12}$ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$ and $\alpha$ = 2.63 $\pm$ 0.05, corresponding to an integrated flux above 1 TeV of 1.97 $\times$ 10$^{-11}$ photons cm$^{-2}$ s$^{-1}$. The systematic error is estimated to be less than 30$\%$ for the flux normalization and 0.06 for the spectral index. Assuming a power law spectrum with an exponential cutoff dN/dE = I$_0$ (E / 2 TeV)$^{-\alpha}$ $\exp$ (-E / E$_{cut}$), the lower limit of the cutoff energy E$_{cut}$ is 12 TeV, at 90$\%$ confidence level. Our extended dataset allows the study of the TeV emission over long timescales. Over five years, the light curve of the Crab Nebula in 200-day bins is compatible with a steady emission with a probability of 7.3 $\times$ 10$^{-2}$. A correlated analysis with Fermi-LAT data over $\sim$4.5 years using the light curves of the two experiments gives a Pearson correlation coefficient $r$ = 0.56 $\pm$ 0.22. Concerning flux variations on timescales of days, a "blind" search for flares with a duration of 1-15 days gives no excess with a significance higher than four standard deviations. The average rate measured by ARGO-YBJ during the three most powerful flares detected by Fermi-LAT is 205 $\pm$ 91 photons day$^{-1}$, consistent with the average value of 137 $\pm$ 10 day$^{-1}$., Comment: 12 pages, 10 figures