The afterglow of gamma ray bursts II: the cases of GRB 970228 and GRB 970508

Highly radiative expansion of a relativistic shell is shown to explain all observed features of the afterglows of the two bursts GRB 970228 and GRB 970508. In particular, in the first case the observed time-dependence t^-1.32 of the soft X--ray flux is easily reproduced. The same model, when the surrounding matter density scales as a r^-2, explains the afterglow of GRB 970508}, which may at first sight appear at odds with that of GRB 970228. In particular, it is shown that both the late peak in the optical luminosity and the flat time-dependence of the X--ray luminosity are simultaneously explained by nonuniformity of the surrounding matter, that the observed optical time-delay is correctly reproduced for standard parameter values, and that the time-delay and flux levels of the radio emission are also explained.