Multi-wavelength studies of GRB afterglows

We present the discovery of SN 1998bw in the error box of GRB 980425. Our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties. We also present multi-wavelength observations of several gamma-ray burst afterglows: (i) the optical light curves of GRB 970228, GRB 970508, GRB 980326, and GRB 980703 (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.