4.2.1. General Properties and Multi-Wavelength Observations

The extremely bright prompt emission of this event, which is the seventh in brightness among the GRBs detected by the Fermi-GBM until then, lasted for *T*<sup>90</sup> = 48.9 ± 0.4 s and released an isotropic-equivalent energy *<sup>E</sup>γ*,*iso* <sup>=</sup> 6.0 <sup>±</sup> 0.1 <sup>×</sup> <sup>10</sup><sup>53</sup> erg in the 50–300 keV range.

Multi-wavelength afterglow observations covered the entire electromagnetic spectrum (see Figure 11). A significant signal was detected by Fermi-LAT from the trigger time up to 700 s, with the highest photon energy of 5 GeV detected 137 s after the burst trigger [160]. The Swift-XRT telescope observed and identified a bright afterglow starting from 90 s. This was still visible almost 30 days after the trigger time. The late-time light curve (from <sup>2</sup> <sup>×</sup> <sup>10</sup><sup>3</sup> s to <sup>4</sup> <sup>×</sup> <sup>10</sup><sup>6</sup> s) can be modeled with an initial power-law decay with an index <sup>−</sup>1.19+0.01 <sup>−</sup>0.02 followed by a break at *<sup>t</sup>break* <sup>=</sup> <sup>8</sup> <sup>×</sup> <sup>10</sup><sup>4</sup> s to an index of <sup>−</sup>1.55+0.04 −0.05 5 . Several optical observations [161–170] revealed the presence of a counterpart and allowed us to estimate the redshift value of *z* = 0.654. The optical afterglow was observed to be slowly fading at an almost constant rate from around 10–11 h after the trigger time [171,172] as discussed in [173]. Radio observations (not shown in the figure) were also performed starting from ∼1.7 days after the burst showing a steep power-law decaying emission [173].
