Multi-wavelength studies of GRB afterglows

We present multi-wavelength observations of several gamma-ray burst afterglows: (i) the optical light curves of GRB 970228, GRB 970508 and GRB 980326; (ii) the GRB 970508 radio 1.4 GHz and mm (86 GHz) lightcurves, and (iii) the X-ray to radio spectrum of GRB 970508 on May 21 (12.1 days after trigger). A successful and consistent description of afterglow observations is given by a blast wave model, in which a significant fraction of the electrons cool rapidly. For GRB 970508 it succesfully describes the observations ranging in time from trigger until several months later, over 9 decades in frequency from radio to X rays.