The cosmological era

Introduction The observations of gamma-ray bursts (GRBs) with the Compton Gamma Ray Observatory /Burst and Transient Source Experiment ( CGRO/BATSE ; see Chapter 3) pointed toward an extragalactic origin of GRBs; however, a direct measurement of the distance was still missing. Faster and much more precise positions were needed to find the counterparts at other wavelengths, the “Holy Grail” of GRB scientists. However, whether any “remnant” (afterglow) emission actually followed the GRB was unknown. A number of breakthrough discoveries were enabled by the launch of BeppoSAX in 1996. The fundamental questions about the nature of GRBs addressed with BeppoSAX and their implications, can be summarized as follows: Where are GRBs located? – distance-scale measurement; properties of the host galaxy; properties of the GRB formation site. What is the source of the GRB energy? – nature of their progenitors; how the progenitors produce the initial GRB energy (collapsars or mergers?). How is this energy transformed into radiation? BeppoSAX BeppoSAX was an Italian X-ray satellite with Dutch participation, nicknamed Beppo after the physicist Giuseppe “Beppo” Occhialini (Piro et al. 1995a, Boella et al. 1997a). One of the key science goals of BeppoSAX was the study of transient sources in the sky, including GRBs. Scientific instruments and ground operations were specifically designed to search for transient events over wide areas of the sky, and to follow up their detection with fast observations using sensitive X-ray telescopes.