# Gamma-ray Bursts: 50 Years and Counting!
## 4 The BeppoSAX Era (1996-2000/2004)
Ground-based telescopes corroborated the findings by identifying a fading optical counterpart [27]. These later, fading emissions were called the afterglow of the GRB, to distinguish them from the initial prompt emission, which was the only signal observed up to that moment. Once the event's localization was pinpointed, subsequent deep imaging revealed a faint, very distant host galaxy in the GRB's location. This discovery had far-reaching implications. It swiftly resolved the longstanding debate about the distance scale of GRBs, confirming them as extragalactic events originating within faint galaxies situated at staggering distances. The establishment of this distance scale not only concluded a significant debate but also provided crucial insights into the environments in which GRBs unfold. A few months later, the observation of GRB970508 led to two further breakthroughs. In less than 4 h after its discovery, it was possible to obtain the accurate localization of this event, allowing follow-up observations to begin much earlier than with any previous burst, detecting the optical transient while still on the rise. In addition to the optical component, a radio component of the afterglow emission was observed for the first time [28]. Thanks to the analysis of the absorption lines in the optical spectrum, it was also possible to estimate, for the first time, the distance of the GRB, with a redshift of \(z=0.835\)[29], proving the extragalactic origin of these phenomena. The possibility of measuring the distances of the origins of these signals allowed the discovery of the empirical correlation today known as the Amati relation [30] between the GRB isotropic bolometric emission energy (\(E_{\gamma,\rm iso}\)) and the rest-frame peak energy (\(E_{p,z}=(1+z)E_{p}\)):  
\[\frac{E_{p,z}}{100\,\rm keV}=C\Bigg{(}\frac{E_{\gamma,\rm iso}}{10^{52}\,\rm erg }\Bigg{)}^{m} \tag{3}\]  
with \(C\sim 0.8\)-\(1\) and \(m\sim 0.4\)-\(0.6\)[31]. This correlation holds for long GRBs with known redshifts, and its discovery hinted at the possibility of using GRBs as cosmological probes. In 2004, a correlation (now known as the Yonetoku relation) similar to the Amati relation was highlighted between the isotropic, bolometric peak luminosity \(L_{\gamma,p,\rm iso}\) and the rest-frame peak energy \(E_{p,z}=(1+z)E_{p}\)[32]:  
\[\frac{E_{p,z}}{100\,\rm keV}\approx 1.8\Bigg{(}\frac{L_{\gamma,p,\rm iso}}{10 ^{52}\,\rm erg\,s^{-1}}\Bigg{)}^{0.52} \tag{4}\]  
Although it has been suggested by several groups that the Amati and Yonetoku relations may be due to observational selection effects (e.g., [33]), this conclusion is not widely accepted, with other groups stating that the presence of such selection effects, despite lowering the correlation, are not enough to destroy it completely (e.g., [34]). Another piece of evidence supporting the existence of such correlations could be the existence of a similar correlation between \(L\) and \(E_{p}\) within some bursts themselves [35]. It is interesting to note that short and long GRBs do not share the same Amati relation, with short GRBs having a parallel and higher relation compared to that for the long GRBs, suggesting that they are systematically less energetic than the long GRBs given the same \(E_{p,z}\). This could be attributed to the shorter durations, which would tighten the relationship between luminosity and \(E_{p,z}\) for such phenomena. On the other hand, short and long GRBs are no longer well distinguishable in the \(E_{p,z}-L_{\gamma,p,iso}\) plane, suggesting that the radiative processes responsible for the emissions are the same [36; 37]. The abundant multiwavelength afterglow data allowed an in-depth understanding of the physics of GRBs. The spectral data were found to be generally in agreement with the power-law decay behavior predicted by the fireball forward shock model [38; 39], while early optical flashes were interpreted as reverse shock emission [21; 40].