Abstract
Particle-in-cell simulations of jets of electrons and positrons in an ambient electron–proton plasma have revealed an acceleration of positrons at the expense of electron kinetic energy. We show that a filamentation instability, between an unmagnetized ambient electron–proton plasma at rest and a beam of pair plasma that moves through it at a non-relativistic speed, indeed results in preferential positron acceleration. Filaments form that are filled predominantly with particles with the same direction of their electric current vector. Positron filaments are separated by electromagnetic fields from beam electron filaments. Some particles can cross the field boundary and enter the filament of the other species. Positron filaments can neutralize their net charge by collecting the electrons of the ambient plasma, while protons cannot easily follow the beam electron filaments. Positron filaments can thus be compressed to a higher density and temperature than the beam electron filaments. Filament mergers, which take place after the exponential growth phase of the instability has ended, lead to an expansion of the beam electron filaments, which amplifies the magnetic field they generate and induces an electric field in this filament. Beam electrons lose a substantial fraction of their kinetic energy to the electric field. Some positrons in the beam electron filament are accelerated by the induced electric field to almost twice their initial speed. The simulations show that a weaker electric field is induced in the positron filament and particles in this filament hardly change their speed.
Highlights
Annihilation radiation from the bulge of our galaxy evidences a sizeable positron population.[1,2] The huge mean free path of the interstellar medium means that positrons can travel far without annihilating,[3] and it is difficult to determine their source
We show that a filamentation instability, between an unmagnetized ambient electron–proton plasma at rest and a beam of pair plasma that moves through it at a non-relativistic speed, results in preferential positron acceleration
The simplest model for a pair-plasma microquasar jet expanding into an ambient plasma is a beam of electrons and positrons that flows across an unmagnetized ambient electron–proton plasma
Summary
Annihilation radiation from the bulge of our galaxy evidences a sizeable positron population.[1,2] The huge mean free path of the interstellar medium means that positrons can travel far without annihilating,[3] and it is difficult to determine their source. The interaction between a beam of electrons and positrons and ambient plasma composed of electrons and protons can lead to the formation of leptonic jets with a structure that resembles that of hydrodynamic jets.[9] Previous work has shown that positrons gained energy at the expense of the beam electron energies This energy redistribution was observed for jets in unmagnetized plasmas[10] and in plasma that was permeated by a magnetic field parallel to the jet’s expansion direction.[11] The exact reason for this redistribution has eluded us. The simplest model for a pair-plasma microquasar jet expanding into an ambient plasma is a beam of electrons and positrons that flows across an unmagnetized ambient electron–proton plasma This situation might be realistic if we consider the plasma at the jet’s front, known as its head, once it has left the strongly magnetized coronal plasma near the accretion disk.
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