*4.1. GRB 190114C*

On 14 January 2019, MAGIC detected a very significant (at 50*σ* level) emission between 300 GeV and 1 TeV from the long GRB 190114C [2]. The event, initially detected by *Swift*-BAT and *Fermi*-GBM, was a bright (*E*iso∼<sup>3</sup> <sup>×</sup> <sup>10</sup><sup>53</sup> erg in the 1–10<sup>4</sup> keV energy range), a long (*T*<sup>90</sup> = 362 s as measured by *Swift*-BAT) and quite nearby (*z* = 0.4245) GRB.

Figure 7 shows the timescale of MAGIC follow-up observation: MAGIC received the alert from *Swift*-BAT 22 s after the GRB onset and started observations about 1 min after the GRB trigger under moderate moon conditions and at a relatively high zenith (58◦ ). The GRB was detected with the MAGIC's real-time analysis with a significance of 20*σ* in the first 20 min of observations above an approximate threshold of 300 GeV. Later, the signal was confirmed up to 50*σ*-level in the dedicated offline analyses. The detection was reported as quickly as possible to the astrophysical community to strongly encourage the follow-up of this event at other wavelengths. Due to the timescale of early detection, one of the first questions to be answered was if the emission detected by MAGIC was related to the prompt or to the afterglow phase. While the value of *T*<sup>90</sup> may indicate that such emission belongs to the prompt, detailed spectral and temporal studies of the keV– MeV data show that at ∼*T*<sup>0</sup> + 25 s the properties of such low-energy emission are more in agreement with the ones of the afterglow phase. This is additionally confirmed by the similar temporal decay index between the X-ray (from *Swift*-XRT between 0.1 and 1 keV) and the VHE (between 300 GeV and 1 TeV) energy light curves (Figure 8). The intrinsic spectrum of the GRB is compatible with a power law with spectral index of *α*int = −2 between 0.2 and 1 TeV, with no indication of any break or cutoff beyond those energies at the 95% confidence level. Such a flat spectrum shows that the energy output in the VHE range might be considered relevant, turning out to be comparable to the energy release measured at lower energies. Given the high absorption of the VHE flux by the EBL at the redshift of the GRB, the observed spectrum by MAGIC is rather softer and best described by a power law with index *α*obs = −5.43 ± 0.22. This was tested against different EBL models, resulting in similar spectral indexes compatible within the statistical uncertainties.

**Figure 7.** Light curves for MAGIC and *Swift*-BAT. Vertical solid lines show the times of events related to the MAGIC automatic procedure. Extracted from Ref. [2].

The origin of the emission detected by MAGIC is one of the most critical issues. The similarity of the temporal decay in X-ray and VHE energy light curves suggests that the emission processes might be linked and have the same origin. While the simplest hypothesis is that the processes producing the X-ray and VHE photons are the same, namely, synchrotron emission from relativistic electrons accelerated at the external shock in the afterglow of the GRB, this explanation is ruled out if one takes into account that the detected photons largely exceed the synchrotron burn-off limit (see Section 1). Even assuming a Lorentz factor of ∼1000, which is not typical for GRBs, the maximum energy of the photons produced by synchrotron is at most around 100 GeV, even considering different density profiles of the interstellar medium density. Therefore, it is reasonable to assume that the VHE emission is due to a different process. In addition, extrapolating the lowenergy synchrotron spectrum (from *Fermi*-GBM, *Swift*-XRT, and *Fermi*-LAT data) to VHE range would underestimate the MAGIC flux by approximately one order of magnitude, strengthening the conclusion that the VHE photons are actually produced by a different mechanism. However, the existence of a synchrotron burn-off limit intimately assumes that the radiation came from one single emission region. Having more emission regions might allow synchrotron photons to reach higher energies. In the assumption that the VHE emission is not due to synchrotron, the most simple alternative is the SSC.

**Figure 8.** Radio to gamma-ray multi-wavelength light curves of GRB 190114C. The dashed vertical line marks the end of the prompt emission phase. Reprinted with permission from Ref. [76].

The SSC scenario, as commonly observed in other sources such as blazars, foresees a spectral energy distribution (SED) characterized by two distinct emission peaks, one at low energies (X-ray band) due to synchrotron emission, and a second one at higher energies, often in the VHE energy range. The modeling of the GRB 190114C multi-wavelength data with a synchrotron plus SSC emission within the external shock scenario in the afterglow shows exactly these two-peaks features, confirming the presence of an emission component at VHE never observed before (Figure 9). Another remarkable result is the fact that the parameters describing the broadband emission of GRB 190114C have values similar to the ones found in previous studies of GRB afterglows when data only up to the GeV energies were considered. This may hint to the possibility that VHE emission from SSC may be present in all GRBs and that it could be detected by IACTs if favorable conditions apply, i.e., a low enough redshift and good observing conditions. This hypothesis can be confirmed only with the detection of more GRBs in the VHE band.

**Figure 9.** Synchrotron and SSC modeling of the broadband spectra of GRB 190114C in the two time intervals 68–110 s and 110–180 s. Dashed lines represent the SSC emission in the hypothesis of negligible internal *γ* − *γ* opacity. MAGIC points in the VHE band are reported with (empty circles) and without (filled circles) correction for the EBL absorption. From [76].
