# Gamma-ray Bursts: 50 Years and Counting!
## 6 The Swift Era (2004-Ongoing)
Among the proposed models, we mention the fragmentation of the star during the collapse [85] and/or the fragmentation of the accretion disk due to gravitational instability [86] in the black hole-torus/accretion disk model. Alternatively, these spikes could be produced by the magnetic activity of a rapidly spinning neutron star (magnetars) [87; 88]. Thanks to its high performance, _Swift_ was able to greatly expand the redshift range in which GRBs were observed: at low redshifts, several \"low-luminosity\" GRBs were observed, leading to the hypothesis that they may form a distinct population compared to \"high-luminosity\" GRBs [89], while at high redshifts, _Swift_ broke the record several times over the years with different observations. The first record-breaking GRB observed was GRB050904 at a redshift \(z=6.29\)[90; 91], discovered about a year after the mission's launch. Other notable GRBs were GRB080913 at \(z=6.7\)[92] and GRB090423 at \(z=8.2\)[93; 94] (still the most distant GRB with strictly spectroscopic redshift estimation) and GRB090429B, which, at \(z=9.4\), remains the most distant GRB ever observed (with photometric redshift estimate) [95]. The increasing number of GRBs with known redshifts allowed the parameters of the Amati relation to be finely calibrated [31] and led to the discovery of two new empirical correlations between the observational parameters. Some \"anomalous\" signals observed by _[PERSON]_ led, for the first time, to the question of whether the distinction between long and short GRBs is intrinsically linked to different progenitor populations. In 2006, _[PERSON]_ had already detected two long-duration GRBs (GRB060614 and GRB060505, [96]) that had temporal lags and peak luminosities that did not fit with those expected for long GRBs but were perfectly reasonable for short GRBs. Moreover, deep optical observations never found any supernovae associated with these emissions, putting very strong constraints on the possibility of an association. These events challenged both the collapsar and the merging neutron star theoretical models for long and short GRBs, suggesting that these long-duration events could be part of a subgroup of the category of short GRBs [96; 97]. This first suggestion of the blurring of the two classes of GRBs was reinforced in the following years by the detection of several short-duration (or \"rest-frame\" short) bursts found to be more consistent with the long-duration population (e.g., GRB080913, GRB090423, GRB090426) [92; 98]. All these findings led to the now-accepted conclusion that the duration criterion alone is insufficient to determine the physical category of a particular GRB, and other criteria must be taken into consideration [36]. Other exceptional events have, in time, appeared, challenging our models and understanding with unexpected characteristics. Some notable examples follow. GRB060218 (and SN2006 aj) was a GRB with unusual characteristics never seen before: in fact, it had a duration of almost 2000 s, much longer than the typical gamma-ray bursts seen previously, and its host galaxy was identified at a distance of only 440 million light years, much closer than all the bursts previously observed. Furthermore, despite its proximity, the burst was much dimmer than the usual high-luminosity GRBs. Moreover, a smooth light curve, a thermal X-ray component in the time-resolved spectra, and a puzzling UV emission posed strong challenges to the theoretical models [99]. The aforementioned GRBs 060505, 060614, 080913, 090423, and 090426 have led to the understanding that the simple long-short classification scheme is not enough to completely describe the physical origin of GRBs. GRB080319B, also known as the \"Naked eye Burst\", was the first GRB to have a prompt optical emission visible to the naked eye: with a peak visibility at an apparent magnitude of 5.3, it remained visible to the human eye for about 30 s, breaking the record for being the object observable with the naked eye coming from the largest distance [100].