Positrons from pulsar winds

Abstract

Pulsars, or more generally rotation powered neutron stars, are excellent factories of antimatter in the Galaxy, in the form of pairs of electrons and positrons. Electrons are initially extracted from the surface of the star by the intense rotation induced electric fields and later transformed into electron-positron pairs through electromagnetic cascading. Observations of Pulsar Wind Nebulae (PWNe) show that cascades in the pulsar magnetosphere must ensure pair multiplicities of order 10 4 − 10 5 . These pairs finally end up as part of the relativistic magnetized wind emanating from the pulsar. The wind is slowed down, from its highly relativistic bulk motion, at a termination shock, which represents the reverse shock due to its interaction with the surrounding ejecta of the progenitor supernova. At the (relativistic) termination shock, acceleration of the pairs occurs, as part of the dissipation process, so that the cold wind is transformed into a plasma of relativistic non-thermal particles, plus a potential thermal component, which however has never been observed. As long as the pulsar wind is embedded in the supernova remnant these pairs are forced to escavate a bubble and lose energy adiabatically (because of the expansion) and radiatively (because of magnetic and radiation fields). We discuss here the observational constraints on the energy and number content of such pairs and discuss the scenarios that may allow for the pairs to escape in the interstellar medium and possibly contribute to the positron excess that has recently been detected by the PAMELA satellite. Special attention is dedicated to the case of nebulae surrounding high speed pulsars, observationally classified as Pulsar Bow Shock Nebulae. The pairs produced in these objects may be effectively carried out of the Supernova Remnant and released in the Interstellar Medium. As a result, Bow Shock Pulsar Wind Nebulae might be the main contributors to the positron excess in the Galaxy.

Particle acceleration and radiation friction effects in the filamentation instability of pair plasmas

“…In particular the excitation of unstable modes by the accelerated particles propagating into the interstellar medium has been studied as a possible mechanism to generate strong magnetic turbulence predicted by the non-linear diffusive shock acceleration theory (see the reviews by Malkov & Drury (2001) and Blandford & Eichler (1987)). The study of a model problem characterized by two countestreaming electron-positron plasma clouds at relativistic energies can be relevant to various astrophysical scenarios including the fireball model of Gamma Ray Bursts (Piran 2005), pulsar wind outflows in Pulsar Wind Nebulae (Blasi & Amato 2011), and relativistic jets from Active Galactic Nuclei (Begelman et al 1984). In this context, several authors have studied counterstraming pair plasmas in various configurations (see e.g.…”

Kadomtsev-Petviashvili solitons propagation in a plasma system with superthermal and weakly relativistic effects

“…38 Our results may be helpful in the understanding of the nonlinear dynamics of astrophysical and laboratory produced plasmas such as pulsar relativistic wind and laser plasma high energy processes (collision of relativistic pulsar wind with supernova ejecta, interaction of intense laser beams with plasma) giving rise to the production of relativistic and superthermal particles in plasmas. [18][19][20] V. SUMMARY Electron-positron-ion plasmas with the relativistically streaming ions and superthermal electrons, positrons are a well established idea. Such plasmas occurs in space and astrophysical environments as well as in the laser plasma interaction experiments.…”

Vela-X as main contributor to the electron and positron spectra at energy above 100 GeV

“…These objects identify the region around the pulsars where a relativistic magnetized wind is populated with electron and positron pairs [12,13] and they are characterised by a very wide photon spectrum supposed to be produced by these particles via synchrotron and inverse Compton processes.…”