Cosmic Gamma-Ray Bursts

Since their discovery in 1973, Gamma-Ray Bursts (GRBs) have remained for many years one of the most elusive mysteries in High Energy-Astrophysics. Despite the fact that ∼800 GRBs are detected every year in the full sky, only a few of them can be localized accurately to less than half a degree. Thus the main problem regarding the nature of GRBs has usually been the lack of knowledge of their distance scale. 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 BSAX and RXTE 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 has led to the discovery of the first counterparts at other wavelengths, greatly improving our understanding of these puzzling sources. Now it is widely accepted that GRBs originate at cosmological distances but the central engines that power these extraordinary events remain still unknown.