4.3.1. General Properties and Multi-Wavelength Observations

The duration of the prompt emission is *T*<sup>90</sup> ≈ 116 s as measured by Fermi-GBM and *T*<sup>90</sup> ≈ 362 s by Swift-BAT. However, the prompt light curve showed a multi-peak structure only for about 25 s, suggesting that the remaining activity, which is characterized by a smooth power-law decay and recorded by these instruments, may already be the afterglow emission. Support to such an interpretation is also obtained from a joint spectral and temporal analysis of the Fermi-GBM and Fermi-LAT data [178]. The total radiated prompt energy is *<sup>E</sup>γ*,*iso* = (2.5 <sup>±</sup> 0.1) <sup>×</sup> <sup>10</sup><sup>53</sup> erg in the energy range 1–10<sup>4</sup> keV [179].

Extensive follow-up observations from several different instruments from GeV to radio are available. Light-curves are shown in Figure 17. Fermi-LAT observations started since the beginning of the prompt phase. A GeV counterpart was detected from *T*<sup>0</sup> to 150 s, when the burst left the LAT field of view and remained outside it until 8600 s. When LAT resumed observations, a significant signal was still detected at a flux level <sup>∼</sup><sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>10</sup> erg cm−<sup>2</sup> s −1 (0.1–1 GeV). After ∼60 s from the burst trigger, Swift-XRT started follow-up observations, which covered in total <sup>∼</sup>10<sup>6</sup> s. The light-curve in the 1–10 keV energy band is consistent with a power-law decay *F* ∝ *t <sup>α</sup>* with *<sup>α</sup>* <sup>=</sup> <sup>−</sup>1.36 <sup>±</sup> 0.02 [71]. NuSTAR and XMM-Newton observations are also available around 1–2 days. The NIR, optical and UV data were taken from around ∼100 s. The early emission is particularly bright and is interpreted as dominated by the reverse shock component [180]. Afterwards, the decay rate flattens and then steepens again after <sup>∼</sup><sup>3</sup> <sup>×</sup> <sup>10</sup><sup>4</sup> s (see Figure 17). The Nordic Optical Telescope measured a redshift of *z* = 0.4245 ± 0.0005 [181], which was then confirmed by Gran Telescopio Canarias [182]. Radio and sub-mm data were taken from <sup>∼</sup>10<sup>4</sup> s and exhibit an achromatic behavior, possibly dominated by the reverse shock in the sub-mm range, followed by emission at late times with nearly constant flux.

**Figure 17.** GRB 190114C: light-curves at different frequencies. From [71].

4.3.2. VHE Observations and Results

After receiving (at 22 s after the BAT trigger time) and validating (at 50 s) the GRB alert, the MAGIC telescopes started observing GRB 190114C at 57 s and operated stably from 62 s, starting from a zenith angle of 55.8◦ . Observations lasted until 15,912 s, when a zenith angle of 81.14◦ was reached. The observation was performed in good weather conditions but in presence of the moon, resulting in a night sky background approximately six times higher than the standard dark night conditions. The results of the offline analysis demonstrate a clear detection above the 50*σ* level in the first 20 min of observation [3].

The light-curve (see Figure 18, upper panel) for the intrinsic flux (i.e., corrected for the EBL absorption) in the 0.3–1 TeV range was derived starting from 62 s and up to 2454 s. The TeV light curve is well described by a power-law with temporal decay index *β<sup>T</sup>* = −1.60 ± 0.07, steeper than the one exhibited by the X-ray flux. The temporal evolution of the intrinsic spectral photon index *αint* of the TeV differential photon spectrum is shown in the bottom panel. A constant value of *αint* ≈ −2 is consistent with the data, considering the statistical and systematic errors, but there is evidence for a softening of the spectrum with time. The spectral fit in the 0.2–1 TeV energy range for the time-integrated emission (62–2454 s) returns *<sup>α</sup>obs* <sup>=</sup> <sup>−</sup>5.34 <sup>±</sup> 0.22 and *<sup>α</sup>*int <sup>=</sup> <sup>−</sup>2.22+0.23 <sup>−</sup>0.25 for the observed and EBL-corrected spectrum, respectively.

**Figure 18.** GRB 190114C. Upper panel: MAGIC light-curve (red circles), compared with the XRT (green band) and LAT (red band) emission. Bottom panel: temporal evolution of the intrinsic spectral photon index in the MAGIC data analysis time bins. From [3].
