Multiwavelength observations of gamma-ray bursts: Towards the understanding of the mystery

Since their discovery in 1973, Gamma-Ray Bursts (GRBs) have remained for many years one of the most elusive mysteries in High Energy-Astrophysics. The main problem regarding the nature of GRBs has usually been the lack of knowledge of their distance scale. About 300 GRBs are detected annually by BATSE in the full sky, but only a few of them can be localized accurately to less than half a degree. For many years, follow-up observations by other satellites and ground-based telescopes were conducted, but no counterparts at other wavelengths were found. The breakthrough took place in 1997, thanks to the observation by BeppoSAX and RossiXTE of the fading X-ray emission that follows the more energetic gamma-ray photons once the GRB event has ended. This emission (the afterglow) extends at longer wavelengths, and the good accuracy in the position determination by BeppoSAX has led to the discovery of the first optical counterparts -for GRB 970228, GRB 970508, and GRB 971214-, greatly improving our understanding of these puzzling sources. Now it is widely accepted that most bursts originate at cosmological distances but the final solution of the GRB problem is still far away.