4.2.3. Interpretation

The H.E.S.S. Collaboration explored two possible radiation mechanisms to explain the VHE emission from GRB 180720B [4]: synchrotron emission and SSC radiation. A full modeling is not performed, and the discussion and comparison among the two different scenarios is based on estimates of the typical and maximal electron energy necessary in the two cases and on the comparison between spectral and temporal indices in different energy ranges. A synchrotron spectrum with a flat (*α* ∼ −2) slope extending from X-ray to VHE could model the emission with one single broad component and explain the similarity between the H.E.S.S., Fermi-LAT, and Swift-XRT luminosities, and the consistency among their photon index values. The large error on the VHE photon index, however, is not placing strong constraints, leaving open both the possibility of a consistency with the extrapolation of the synchrotron spectrum but also the possibility of a spectral hardening, indicative of a second component. A synchrotron origin of 10<sup>2</sup> GeV photons would require to find a process able to accelerate electrons up to PeV energies, which is in excess of the maximum electron energy achievable in external shocks (for a discussion, see Section 2.3.1). Adopting the standard Bhom limit, >100 GeV emission 10 h after the burst would require a huge bulk Lorentz factor Γ ∼ 1000, which at these late times is really unlikely. As a result, these strong requirements disfavor the synchrotron emission as responsible of the VHE component in GRB 180720B.

The SSC scattering, on the contrary, arises as a natural candidate. A full broad-band modeling of GRB 180720B data in this scenario is presented in [174]. A numerical code

reproducing the synchrotron and SSC emission in the afterglow shocks has been used (see [175]). The resulting light-curves and SED in the H.E.S.S. observational time window are shown in Figure 16. The full emission is explained as afterglow forward shock radiation (except for the initial peak in the optical and X-ray curves at *<sup>t</sup>* <sup>∼</sup> <sup>10</sup><sup>2</sup> s, which is attributed to reverse shock emission). In the case of a constant-density ISM environment, the parameters that best reproduce the data are: *E<sup>k</sup>* = 10<sup>54</sup> erg, *n* = 0.1 cm−<sup>3</sup> , *e<sup>e</sup>* = 0.1, *e<sup>B</sup>* = 10−<sup>4</sup> , Γ<sup>0</sup> = 300 and *p* = 2.4. As it can be noticed, the equipartition factor *e<sup>B</sup>* needs to assume quite a low value in order to explain the observations. A stellar wind-like environment is discarded by the authors on the basis of the comparison between the expected flux at <sup>∼</sup>1–10 GeV, following the prescriptions of [48] (& <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>6</sup> erg cm−<sup>2</sup> s <sup>−</sup><sup>1</sup> at *<sup>t</sup>* <sup>≈</sup> <sup>100</sup> s) and the one observed by Fermi-LAT (∼10−<sup>8</sup> erg cm−<sup>2</sup> s <sup>−</sup><sup>1</sup> at *<sup>t</sup>* <sup>≈</sup> <sup>100</sup> s). A low magnetic field equipartition factor is derived from the condition *EKN* & 0.44 TeV at *t* ≈ 10 h, where *EKN* is the energy at which the KN scattering becomes relevant. A transition energy between the synchrotron and SSC component of ∼1 GeV is derived. Such value falls into the Fermi-LAT energy range and is compatible with a hardening of the spectrum in the VHE band. However, since the Fermi-LAT sensitivity is above the predicted flux of GRB 180720B at 10 h in the GeV band, the data cannot firmly confirm the presence of this transition.

**Figure 16.** GRB 180720B: modeling of the broad-band light curves (**left** panel) and SED at the time of the H.E.S.S. detection (**right** panel) proposed by [174]. Both the synchrotron and the SSC contribution to the total flux are shown (see legend). In the SED, X-ray and H.E.S.S. data are shown, respectively, with the green and the blue boxes.

#### *4.3. GRB 190114C*

GRB 190114C is a long GRB at redshift *z* = 0.42 triggered by the Swift-BAT [176] on 14 January 2019 at *T*<sup>0</sup> = 20 : 57 : 03 UT, and by the Fermi-GBM [177]. The event was detected also by several other space *γ*-ray instruments such as AGILE, INTEGRAL/SPI-ACS, Insight/HXMT, and Konus-Wind [71]. MAGIC detected GRB 190114C starting ∼60 s after *T*<sup>0</sup> with a significance above 50*σ*.
