4.5.1. General Properties and Multi-Wavelength Observations

The (isotropic equivalent) prompt emission energy inferred from spectral analysis of Fermi-GBM data is *<sup>E</sup>γ*,*iso* = (1.1 <sup>±</sup> 0.2) <sup>×</sup> <sup>10</sup><sup>50</sup> erg [202]. The prompt duration is *<sup>T</sup>*<sup>90</sup> <sup>=</sup> 9.78 ± 3.47 s (15–350 keV band). The BAT time-average spectrum in the time interval 0–10 s is well fitted by a power-law model with a photon index *β<sup>T</sup>* = −3.03 ± 0.68, suggesting a low peak energy *E<sup>p</sup>* < 10 keV [203].

Swift-XRT [204] follow-up the event starting only 3214 s after *T*<sup>0</sup> due to observational constraints. The light curve up to almost 1 day is well described by a power-law with decay index *<sup>α</sup>* <sup>=</sup> <sup>−</sup>1.49+0.24 <sup>−</sup>0.21. Late-time observations performed by the Chandra X-ray Observatory [205] and Swift-XRT [206] from ∼8 days up to ∼21 days showed a flattening of the X-ray light curve, i.e., a flux level inconsistent (higher) with the extrapolation of the power-law decay rate at early times. Optical observations confirmed the presence of an afterglow counterpart from around 168 s [207]. The optical light-curves showed a clear initial rise with a peak around 200 s followed by a decay [208]. A bright radio counterpart (flux density <sup>∼</sup>1.3 <sup>×</sup>10−<sup>4</sup> Jy at 6 GHz 1.4 days after the burst) was also detected by several instruments [209–211]. Late-time optical observations identified an associated supernova rising from 5 days after the burst reaching its maximum flux around 12–20 days after *T*<sup>0</sup> [212,213]. The measurement of the redshift was reported by the GTC (*z* = 0.426) [214] and then confirmed by the NOT (*z* = 0.423) [215] instrument.
