Nonlinear instability in relativistic pair beams: magnetic field amplification and electron-positron asymmetry

Relativistic electron-positron pair beams are often produced in gamma-ray bursts and blazar jets through photon-photon collisions. These beams can become unstable to plasma microinstabilities, however when the beam is less dense than the ambient medium, these instabilities saturate at small spatial scales and only a small fraction of the beam kinetic energy is converted to magnetic fields. We present a new nonlinear instability which can grow after saturation of the Weibel instability and increases the magnetic field strength and spatial scale by orders of magnitude, allowing magnetic fields to reach near-equipartition levels even at very low beam densities. In addition, the instability leads to an energy asymmetry between the beam species; the beam electrons drive the formation of magnetic-field-filled cavities in which they are inductively decelerated, while the positrons are repelled and flow through the cavity walls. This preferential deceleration of electrons can have important consequences for matter-antimatter asymmetries in astrophysical sources. In addition, in GRBs, the nonlinear instability may dominate magnetic field amplification upstream of relativistic shocks with important consequences for particle acceleration and radiation emission.