Multiwavelength chase of GRB 031220 afterglow

Several gamma ray bursts (GRBs) with X-ray afterglow do not show any optical-IR afterglow. The nature of this class of events, the so-called Dark Bursts, is still not clear. The optical absorption could be due to the interstellar dust or to the high redshift of the event. Or, more simply, the non-detect ion of the optical transient, should be due to the delay in the observation or to the rapid energy decaying of these events. High spatial resolution X-ray observations are the most promising tool to investigate on such kind of events. We have collected and analyzed X-ray data and images taken in different spectral hands (optical and infrared) for GRB 031220 and we present the results of the analysis of multiband observations on the field of this burst. Comparison between images taken at different epochs in the same filters (lid not reveal any strongly variable sources. Photometric analysis and photometric redshift estimation of all possible afterglow candidates suggest that this GRB call be classified as a Dark Burst.