Item

Abstract

Many astrophysical sources are very efficient particle accelerators. Among these, PulsarWind Nebulae (PWNe) are excellent laboratories to study the acceleration mechanism because of their proximity and brightness. In these objects, the rotational energy extracted from the central pulsar is converted in a striped-wind consisting of relativistic electrons and positrons and large-amplitude electromagnetic fields. When the wind impacts with the surrounding medium, a relativistic shock forms where the ordered flow of the pulsar becomes randomised and energetic particles emit Synchrotron and Inverse Compton radiation. We study the conversion of the pulsar wind in a strong superluminal wave. The conversion is triggered by the interaction of the wind with the shock when this is sufficiently far away from the pulsar. The shock structure is modified by the superluminal wave and the properties of the modified shock (precursor) depend on the Lorentz factor and magnetisation of the incoming wind. A relatively large fraction of electrons and positrons in the wind is energised and reflected by the turbulent electromagnetic fields in the precursor and is available for further acceleration. We suggest that the onset of the precursor is a solution to the problem of injection in the acceleration mechanism. We investigate the acceleration of the injected particles and we find that it proceeds in two distinct regimes, determined by the relative magnitude of the wavelength of the wind stripes and of the gyro-radius of energetic particles. We discuss the implications of this acceleration scenario in the context of PSR B1259-63, a pulsar powering a PWNe in a binary system.