# Gamma-ray Bursts: 50 Years and Counting!
## 6 The Swift Era (2004-Ongoing)  
In the early 2000s, the GRB phenomenon seemed almost completely decoded. BATSE observations had led to excellent modeling of gamma emissions, identifying two subpopulations of GRBs depending on their duration. Subsequent observations by BeppoSAX and HETE-2 led to the identification of multiwavelength afterglows and, through accurate localization, allowed long GRBs to be correlated with supernova emissions. These discoveries were accompanied by impressive and rapid theoretical advances that led to the development of, among others, the internal/external shock fireball model and the possible central engines of GRBs. Still, there were some \"details\" of these phenomena that were not yet well understood. First of all, even though BeppoSAX had been able to observe a large number of GRB afterglows, these were just from long-duration bursts due to the requirement to slew the telescopes to observe the signals with the main cameras once the wide-field cameras were triggered. This operation, carried out from the ground, was very slow and therefore limited the observation possibilities only to long-duration GRBs and meant that there was a gap in the data acquired between the prompt-emission signals that triggered the wide-field cameras and afterglow observations after slewing. To fill these observational gaps, the Neil Gehrels Swift Observatory mission [65] was launched on 20 November 2004. _Swift_ is an international space observatory with five main objectives: to quickly (automatically, in \(\sim\)1-2 min) repoint its NFIs in order to study the early afterglow emissions (and better understand both the afterglow onset and the connection with the prompt-emission phase), to observe GRBs across a large spectral band, to collect a large sample of GRBs and afterglows, and to study short GRBs and to trigger ground-based follow-up observations in order to perform accurate spectroscopy measurements. To achieve these objectives, _Swift_ hosts three instruments on board. The Burst Alert Telescope (BAT) [66] is a coded-aperture-mask telescope sensitive between 15 and 150 keV capable of localizing the position of the signals with an accuracy of 1 to 4 arcminutes within 15 s over a 1.4-steradian field of view. This instrument is used to quickly localize the positions of the GRBs to slew the telescope in order to point the NFIs and simultaneously send the localization to the ground through the GCN system to trigger follow-up observations. The X-ray Telescope (XRT) [67] is an X-ray focusing telescope sensitive between 0.3 and 10 keV capable of taking images and light curves and performing a spectral analysis of the GRB afterglows. This instrument also allows the refinement of the BAT localizations of GRBs, with a typical error radius of approximately 2 arcseconds. The Ultraviolet/Optical Telescope (UVOT) [68] is a 30 cm Ritchie-Chretien UV/optical telescope used to detect GRBs' optical afterglows, providing localization with \(<\)arcsecond precision and performing optical and ultraviolet photometry through the use of lenticular filters and low-resolution spectra (170-650 nm) thanks to optical and UV prisms. UVOT can also provide long-term follow-up of the afterglow light curves of GRBs. The combination of these instruments allowed a progressive refinement of the localizations of the signals. First, BAT is triggered by the GRB signal and calculates the position to \(<\)4 arcmin. Then, while the first localization is sent to the ground through the GCN network, the spacecraft autonomously slews to the GRB position in 20-70 s. XRT and UVOT then start their observations, determining the position to \(<\)5 arcseconds, and then transmit their data to the ground. In this way, _Swift_ manages to identify on average 100 new GRBs every year, of which the vast majority are long GRBs. _Swift_ has obtained (and continues to obtain) exceptional results in this way, which have revolutionized our understanding of GRBs in several ways thanks to some fundamental discoveries.