6 Decay law

In Fig. 1 we show the fluxes of the well established afterglow sources. Since the limit of detectability is of $\rm \approx 5\ 10^{-14} \, erg\, cm^{-2}\, s^{-1}$ most of the detected afterglow sources span in one decade of flux. GRB 970508 shows the above mentioned complexities but the majority of the bursts detected with a reasonable statistics, decay with a plain power law and the statistics does not allow for any detail in the light curve. We can state that the slope of the decay law is more differentiated than the energy spectrum. The situation is complicated by intrinsic limitations of BeppoSAX that cannot be overcome. For the majority of bursts, we can determine the slope of the decay with a reasonable sensitivity only by using the values of the flux measured by Wide Field Cameras soon before the flux sinks below the threshold of detectability (around $\rm 10^{-9}\, ergs\, cm^{-2}\,s^{-1}$) and making an assumption on the start of the afterglow. WFC cannot detect any more the GRB after a few $\rm \approx 10^{2}\, s$ and the Narrow Field Instruments cannot be on source in less than 5 hours. In a few cases this assumption is supported by the spectral evolution (Frontera et al. 1998) but in most cases the assumption on the start of the afterglow and, thence, the slope derived by this way, may include a large amount of discretion.

From a preliminary analysis some bursts (e.g. 970228, 980613) seem to have a gap between the prompt burst and the afterglow. In some others (e.g. 970508, 971214 or 980329) we find hints of a continuous evolution from the GRB to the afterglow. This is relevant not only to understand details of models but even to determine the total energy in the afterglow. Luckily our ignorance is not complete thanks to a few serendipitous detections by other satellites. Few data from SIGMA and HEAO1 enlighten this point. One case confirms that a GRB can continuously evolve to the afterglow (Burenin et al. 1999) while another (Connors & Hueter 1999) confirms that a gap may exist after the GRB vanishes and before the afterglow epiphany, and this is consistent with SAX results.