# Gamma-ray Bursts: 50 Years and Counting!
## 4 The BeppoSAX Era (1996-2000/2004)
The possibility of analyzing the evolution of the afterglow spectrum over time led to the proposal that GRB emission occurs in highly collimated jets: the discovery of an achromatic break (optical and radio bands) in the afterglow of GRB990510 (later observed in other GRBs) of around \(t\sim 1\,\rm day\) led, for the first time, to the hypothesis that the emission could occur in the jet rather than isotropically [41]. Analyzing the relatively small sample of afterglows observed until then, it was discovered that, surprisingly, the total jet-corrected energy of the GRB sample is essentially constant, narrowly clustered around \(5\times 10^{50}\,\rm erg\)[42]. This finding was at first interpreted as a hypothesis that different GRBs can collimate a standard energy reservoir into different jet angles. A different explanation was proposed in parallel in [43; 44]: all GRBs have an (almost) universal, structured jet. The differences between the various GRBs may correspond to the different viewing angles of this universal jet such that the measured \"jet angle\" is not the opening angle of a uniform jet, but rather the observer's viewing angle from the jet axis. Thanks to multiwavelength data and the ability to measure the jet angle, it was possible to discover another tight correlation between \(E_{p,z}\) and the beaming-corrected bolometric emission energy \(E_{\gamma}\) ([PERSON]'s relation [45]):  
\[\frac{E_{p,z}}{100\,\rm keV}\approx 4.8\Bigg{(}\frac{E_{\gamma}}{10^{51}\, \rm erg\,s^{-1}}\Bigg{)}^{0.7} \tag{5}\]  
The discovery of these robust correlations led to the proposal to use GRBs as standard rulers for cosmological measurements (e.g., [46]). Thanks to the possibility of carrying out multiband observations of these signals, particularly in the optical range, with excellent localization, it became possible to look for further clues about the nature of the progenitors. The detection of the long-lasting, fading, multiwavelength afterglow emissions allowed the identification of the host galaxies, solving the \"no host problem\" and firmly establishing the origin of GRBs as extragalactic. The ability to obtain accurate localizations showed that the GRBs are located toward the edges of the host galaxies [47; 48], i.e., in the star formation zones [49], and not toward the center. This fitted well with one of the most investigated scenarios, i.e., the supernova-GRB association. The first hint of such an association was the discovery of SN 1998 bw, a type Ic supernova in a galaxy near \(z=0.0085\), in the error box of the BeppoSAX burst GRB980425 [50; 51]. Furthermore, a supernova red bump was discovered in the optical light curves of several other GRBs [52]. These findings led to the development of various models for the progenitors. Refinements of the collapsar model, first proposed by [PERSON] in 1993 [22], have been proposed by various groups. [PERSON]'s group performed the first detailed numerical simulations of a jet launched from a collapsing Wolf-Rayet star, giving robustness to the collapsar model [53]. Many observational features were elegantly explained with this model, such as the presence of a less energetic \"cocoon\" surrounding the jet, as shown in [54]. In 1998, in the same article where he located GRBs within the star-forming regions of their host galaxies, [PERSON] proposed a variant of [PERSON]'s collapsar model, the so-called hypernova model: a rapidly rotating star collapses into a black hole surrounded by a thick accretion disk (or \"torus\"), which magnetically launches a relativistic jet, which, in turn, powers the observed GRB [49]. Another variation of this scenario was proposed in the Black-Hole Accretion Disk (BHAD) Models, where the binary merger of two compact objects or the collapse of a rotating star produces a rapidly accreting disk (\(>\)0.1 solar masses per second) around a \"hyperaccreating\" black hole [55]. Other models went in different directions.