Compton driven beam formation and magnetisation via plasma microinstabilities.

Compton scattering of gamma rays propagating in a pair plasma can drive the formation of a relativistic electron positron beam. This process is scrutinised theoretically and numerically via particle-in-cell simulations. In addition, we determine in which conditions the beam can prompt a beam-plasma instability and convert its kinetic energy into magnetic energy. We argue that such conditions can be met at the photosphere radius of bright Gamma Ray Bursts.