  Electrons in shocks are efficiently energized due to the cross-shock
potential, which develops because of differential deflection of electrons and
ions by the magnetic field in the shock front. The electron energization is
necessarily accompanied by scattering and thermalization. The mechanism is
efficient in both magnetized and non-magnetized relativistic electron-ion
shocks. It is proposed that the synchrotron emission from the heated electrons
in a layer of strongly enhanced magnetic field is responsible for gamma ray
burst afterglows.
