Radiative regimes in gamma-ray bursts and afterglows

We present numerical simulations of gamma-ray bursts arising from external shocks in the impulsive and wind models, including a weak or a strong coupling between electrons and protons plus magnetic fields, and analyze the burst features in each scenario. We follow the dynamics of the ejecta and external medium into the late stages of deceleration in order to study the hydrodynamics of the remnant and the temporal and spectral evolution of the afterglow. A brief comparison with the optical and radio afterglows of GRB 970228 and GRB 970508 is made.