Cosmic gamma-ray bursts: the most energetic phenomenon in the Universe

Gamma-Ray Bursts (GRBs) have remained a puzzle for many high-energy astrophysicists since their discovery in 1967. With the advent of the X-ray satellites BeppoSAX and RossiXTE, it has been possible to carry out deep multi-wavelength observations of the counterparts associated with the GRBs just within a few hours of occurence, thanks to the observation of the fading X-ray emission that follows the more energetic gamma-ray photons once the GRB event has ended. The fact that this emission (the afterglow) extends at longer wavelengths, has led to the discovery of the first optical/IR/radio counterparts in 1997-98, greatly improving our understanding of these sources. Now it is widely accepted that GRBs originate at cosmological distances but while the observed afterglow satisfies the predictions of the relativistic fireball models, the central engines that power these extraordinary events remain still unknown. Detailed results for nine selected GRBs are presented as well as a summary of the GRB counterparts and host galaxies found so far.