Abstract. Based on an analysis of a specific electron trajectory in counterpropagating beams, Bell & Kirk (PRL 101, 200403 (2008)) recently suggested that laboratory lasers may shortly be able to produce significant numbers of electronpositron pairs. We confirm their results using an improved treatment of nonlinear Compton scattering in the laser beams. Implementing an algorithm that integrates classical electron trajectories, we then examine a wide range of laser pulse shapes and polarizations. We find that counter-propagating, linearly polarized beams, with either aligned or crossed orientation, are likely to initiate a pair avalanche at intensities of approximately 10 24 W cm −2 per beam. The same result is found by modelling one of the beams as a wave reflected at the surface of an overdense solid.

Context. More than one hundred pulsars have been detected up to now at GeV energies by the Large Area Telescope (LAT) on the Fermi gamma-ray observatory. Current modelling proposes that the high-energy emission comes from outer magnetospheric gaps, but radiation from the equatorial current sheet that separates the two magnetic hemispheres outside the light cylinder has also been investigated. Aims. We discuss the region outside the light cylinder, the "near wind" zone. We investigate the possibility that synchrotron radiation emitted by thermal populations in the equatorial current sheet of the pulsar wind in this region can explain the lightcurves and spectra observed by Fermi/LAT. Methods. We used analytical estimates as well as detailed numerical computation to calculate the γ-ray luminosities, lightcurves, and spectra of γ-ray pulsars. Results. Many of the characteristics of the γ-ray pulsars observed by Fermi/LAT can be reproduced by our model, most notably the position of these objects in the P −Ṗ diagram, and the range of γ-ray luminosities. A testable result is a sub-exponential cutoff with an index b = 0.35. We also predict the existence of a population of pulsars with cutoff energies in the MeV range. These have systematically lower spindown luminosities than the Fermi/LAT-detected pulsars. Conclusions. It is possible for relativistic populations of electrons and positrons in the current sheet of a pulsar's wind immediately outside the light cylinder to emit synchrotron radiation that peaks in the sub-GeV to GeV regime, with γ-ray efficiencies similar to those observed for the Fermi/LAT pulsars.

The energy lost by a rotation-powered pulsar is carried by a relativistic flow containing a mixture of electromagnetic fields and particles. In the inner regions, this is thought to be a magnetically dominated, cold, electron-positron wind that is well described by the MHD equations. However, beyond a critical radius r cr , the same particle, energy and momentum fluxes can be transported by a strong, transverse electromagnetic wave with superluminal phase speed. We analyze the nonlinear dispersion relation of these waves for linear and circular polarization, and find the dependence of r cr on the mass-loading, magnetization and luminosity of the flow, as well as on the net magnetic flux. We show that, for most isolated pulsars, the wind lies well outside r cr , and speculate that superluminal modes play an important role in the dissipation of electromagnetic energy into nonthermal particles at the termination shock.

This paper will account for the interaction between syntax and semantics/pragmatics in the speech styles of Balinese. The analysis makes use of an explicit representation that accounts for the co-occurrence restrictions on linguistic expressions which are imposed by social information associated with the speech- level system. It is proposed that social information be treated in terms of social predicates and modelled using LFG-style parallel structures. The social predicates are contained in what is called pragmatic-structure (prag-str). It is demonstrated that this approach can account for the plain as well as the (dis)honouring use of linguistic forms in Balinese.