Theoretical aspects of the Fireball scenario

Abstract

Collisionless shocks are a key ingredient of the Fireball scenario. Yet, their formationfrom the encounter of two collisionless plasma shells is not understood from firstprinciples. When the shells interpenetrate, the overlapping region turns unstable,triggering the shock formation. As a first step towards a microscopic understanding of theprocess, we analyze here in details the initial instability phase. On the one hand, 2Drelativistic PIC simulations are performed where two symmetric initially cold pair plasmascollide. On the other hand, the instabilities at work are analyzed, as well as the fieldat saturation and the seed field which gets amplified. For mildly relativistic motions andonward, Weibel modes with ω = 0+iδ govern the linearphase. We derive an expression for the duration of the linear phase in reasonableagreement with the simulations.