3 Discussion and conclusions

GRB 971227 is one of the most puzzling GRB observed by BeppoSAX. We associate the fading X-ray source, 1SAX J1257.3+5924, detected in the BeppoSAX MECS, with the best candidate for the X-ray afterglow of GRB 971227. This X-ray afterglow shows a slow decay behaviour tipical of that afterglows discovered both in optical and in X-rays (e.g. GRB 970228, GRB 970508, etc.). On the contrary no optical counterpart was unambiguosly discovered for this GBR as in the case of GRB showing a faster decay (e.g. GRB 970111 (Feroci et al. 1998), GRB 970402 (Nicastro et al. 1998) etc.). The fact that X-ray, optical and radio afterglow are not always observed pointing toward a GRB region is currently explained invoking environment effects. If the optical detection of Castro-Tirado and collaborators is true the optical decay should be faster than that observed at higher energies. This result is different from any other observed in the GRB afterglows up to now and is not easily explainable in terms of environment effects.

  

Figure 1: X-ray (2-10 keV) decay law of the candidate counterpart of GRB 971227. The power-law index, 1.12, needed for connecting the WFC GRB 971227 mean flux and the 1SAXJ 1257.2+5924 flux

Acknowledgements

This research is supported by the Italian Space Agency (ASI). All authors warmly thank the teams of the BeppoSAX Scientific Operation Center, Operation Control Center and Science Data Center for their support to the GRB program.