3 Results and discussion

From this GRB afterglow sample we find that in general, at early epochs after the GRB (2 days), the SED exhibits a maximum in the X-rays. However, due to the lack of data in the far and extreme ultraviolet range (100 to 3500 Å), and to the large uncertainties on the X-ray spectral slope, we cannot exclude that the SED peak is located between the optical and the X-rays. While the afterglow of GRB 970508 exhibits a smooth and continuously curved spectrum from the radio to the X-ray frequencies, which might suggest the presence of a single emission component (presumably synchrotron radiation; e.g., Sari et al. 1998), the other cases reported in Fig. 1 show local maxima (visible on the reported SEDs as "humps") in the optical/IR range. If intrinsic, these would imply that at least two emission components build up the afterglow radio-to-X-ray continuum. However, they could be the signature of gas and dust absorption within the host galaxy, which increases toward the ultraviolet frequencies. Although, due to the scarcity and irregular sampling of the data, it is extremely difficult to follow in detail the temporal evolution of the SED maximum, the data suggest that for most GRB afterglows the peak moves toward lower frequencies with time. The timescale of this displacement is hard to assess; it could be estimated for future GRBs by simultaneous monitoring in many bands, on long time intervals.