# Gamma-ray Bursts: 50 Years and Counting!
## 6 The Swift Era (2004-Ongoing)
Thanks to the fast automatic slewing, _Swift_ is able to detect the faint X-ray afterglow of short-duration GRBs. This was impossible before _Swift_. One year after the launch, in 2005, it was already possible to identify the host galaxies of some of these short GRBs (e.g., GRB050509B and GRB050709 detected with HETE-2 and GRB050724) and obtain their precise positions within them. The observations turned out to be rather different than expected: while long GRBs are generally found in blue, regular, and highly star-forming host galaxies and are located precisely in the star-forming regions of these galaxies, short GRBs, on the other hand, are hosted mainly by elliptical or irregular galaxies, far from star-forming regions [69; 70; 71]. This led to the suggestion that short GRBs may have different progenitors than long GRBs, not being associated with the death of massive stars but with the coalescence of compact objects (such as neutron stars or black holes). The ability to quickly repoint the telescope and observe the initial stages of the afterglows has led to a true revolution in our understanding of these emissions thanks to the abundant available data [72; 73]. Before _Swift_, GRB afterglows were simply modeled with two power laws, one for the \"initial\" phase and one for the \"post jet-break\" phase. It was therefore expected that by observing the initial phases of the afterglow, the same simple power-law trend would be found to connect up to the peak given by the prompt emission. In addition to the two phases already known, it was found that most GRBs have a complex light curve, with two additional phases connecting the prompt-emission phase with that of the afterglow. The early afterglows show, in addition to the two already-known decay phases, an \"early steep decay\" phase directly connected to the prompt emission [74; 75] and a \"plateau\" phase (or \"shallow decay\") before the onset of the normal decay phase [76; 77]. In nearly half of the GRBs, additional X-ray flares, following the prompt gamma-ray emission, were also discovered [78; 79; 80]. The abundance of data has allowed the development of a phenomenological model for all afterglow light curves, the \"canonical X-ray afterglow lightcurve\" [81; 82]. In addition to these more widespread characteristics, some afterglows present even more particular behaviors, such as the presence in some GRBs of a \"chromatic afterglow\" behavior, where the optical light curve has no break coinciding with the X-ray break time (or vice versa), and in particular, there is no association between the temporal variations in the light curve in the optical and in the X-rays [83]. This evidence, which challenged the standard external shock fireball model, led to the proposal that the afterglow emission could be generated by the superposition of different physical processes. The initial steep-decay phase is the continuation of the prompt emission after it has ended due to emission from high latitudes relative to the observer's line of sight. The plateau phase could be given by an external shock emission with continuous energy injection, which requires either a long-duration central engine or a jet with stratified Lorentz factors. In particular, to justify this plateau phase, the models seem to favor the notion that the central engine could be a (supramassive) millisecond magnetar that, thanks to the rotation, manages to survive the collapse long enough to emit the signal in the early afterglow and then collapses into a black hole (with an extremely rapid decay of the light curve) [84]. And finally, X-ray flares may be internal emissions due to the tail end of central engine activity, similar to how prompt gamma-ray emission is produced. Various models have been proposed to explain such emissions as intermittent activity (or re-ignition) of the GRB's central engine.