Early GRB afterglows from relativistic blast waves in general radiative regimes

We present analytical expressions for the spectral evolution of the early afterglow radiation from gamma-ray bursts in radiative regimes intermediate between the adiabatic and the fully radiative solutions. Our expressions are valid as long as the relativistic electrons responsible for the observed synchrotron emission are in the fast cooling regime and the blast wave is relativistic. We show that even a slight deviation from a perfectly adiabatic evolution results in significant changes in the temporal characteristics of the afterglow emission.