Gamma-ray burst phenomenology, shock dynamo, and the first magnetic fields

A relativistic collisionless shock propagating into an unmagnetized medium leaves behind a strong large-scale magnetic field. This seems to follow from two assumptions: (1) Gamma-ray burst (GRB) afterglows are explained by synchrotron emission of a relativistic shock. (2) The magnetic field cannot exist on microscopic scales only; it would decay by phase-space mixing. Assumption 1 is generally accepted because of an apparent success of the shock synchrotron phenomenological model of GRB afterglows. Assumption 2 is confirmed in this work by a low-dimensional numerical simulation. One may hypothesize that relativistic shock velocities are not essential for the magnetic field generation and that all collisionless shocks propagating into an unmagnetized medium generate strong large-scale magnetic fields. If this hypothesis is true, the first cosmical magnetic fields could have been generated in shocks of the first virialized objects.