*4.3. GRB 180720B and GRB 190829A*

GRB 190829A was detected by the *Fermi*-GBM on 29 August 2019 at 19:55:53 UTC [81] with a second detection by *Swift* that occurred 51 s later [82]. The measured redshift of *<sup>z</sup>* <sup>=</sup> 0.0785 [83] implies a total isotropic energy release of <sup>≈</sup>10<sup>50</sup> erg during the prompt phase in both the *Fermi*-GBM and the *Swift* energy bands. This relatively low value placed GRB 190829A on the lower edge of the GRB energy distribution. Nonetheless, the very close-by distance made it a relatively bright GRB in the *Swift*-XRT band. H.E.S.S. observations were performed, starting at *T*<sup>0</sup> + 4.3 h, once afterglow phase emission had already taken over. Follow-up was performed with the use of the four smaller telescopes of the H.E.S.S. array and at a starting zenith angle of ∼40◦ , corresponding to an energy threshold of ∼170 GeV. The analysis reported a clear detection of a VHE gamma-ray signal during the first night with statistical significance of 21.7*σ* in 3.6 h of observation. Surprisingly, the GRB was also detected for the two forthcoming nights at *T*<sup>0</sup> + 27.2 h and *T*<sup>0</sup> + 51.2 h, with statistical significance of 5.5*σ* and 2.4*σ*, respectively. Figure 10 (left panel) shows the fading VHE signal measured during the three nights of observations. As for GRB 190114C, the X-ray and VHE gamma-ray light curves also show similar decay profiles with a time evolution characterized by a power law of index *α*VHE = 1.09 ± 0.05 and *α*XRT = 1.07 ± 0.09 in the H.E.S.S. and *Swift*-XRT bands, respectively. Despite these similarities, the interpretation of the VHE light curve and spectrum of GRB 190829A within the framework of the standard GRB afterglow emission model (such as for GRB 190114C) showed some tensions. The H.E.S.S. data were collected deep in the afterglow phase in a moment in which the bulk Lorentz factor of the outflow was evaluated to be Γ∼4.7 and Γ∼2.6 for the first and second nights of observation, respectively [22,83]. Thus, radiation of few TeV, such as the one measured by H.E.S.S., besides largely exceeding the synchrotron burn-off limit at these specific times, also results in tension with the synchrotron+SSC scenario. With these values of Γ, the electrons producing the VHE emission likely lie in the Klein–Nishina regime. The corresponding reduction in the inverse Compton cross section would introduce a cut-off and a steepening of the flux at VHE. As it appears from H.E.S.S. results (Figure 11), this expected steepening makes it challenging for SSC models to simultaneously reproduce the observed X-ray and VHE spectra [3]. An intriguing possibility is to introduce a leptonic scenario with no limitation placed on the electron maximum energy (and, correspondingly, no synchrotron burn-off limit) that would allow synchrotron emission to produce VHE photons. Although this scenario reproduces the H.E.S.S. data (Figure 11) considerably better, it would require a significant re-evaluation of the relativistic shock-accelerated models. On the other hand, although alternative interpretations have been presented (see, e.g., [84,85]), attempts to model GRB 190829A afterglow using a leptonic synchrotron + SSC emission model have been reported with convincing results and with an obtained set of the shock's microphysical parameters that are similar to those found for GRB 190114C [86].

**Figure 10.** The H.E.S.S. sky map centered at the coordinates of GRB 190829A. The VHE signal was clearly detected for three consecutive nights up to *T*<sup>0</sup> + 51 h. Reprinted with permission from Ref. [3].

**Figure 11.** Multi-wavelength models of the first two nights of GRB 190829A H.E.S.S. The black region represents the spectrum and uncertainty of the *Swift*-XRT observations, while the red region is the H.E.S.S. intrinsic spectrum and its uncertainty. The green arrow is the upper limit set by observation during the first night by *Fermi*-LAT. The used model is a standard SSC model with 68% confidence intervals determined from the posterior probability distribution of the model's parameter fitting for the synchrotron component (orange) and the SSC one (blue). No synchrotron cut-off energy (burn-off limit) is considered. Reprinted with permission from Ref. [3].

GRB 180720B was detected by *Fermi*-GBM at 14:21:39.65 UTC [87], and 5 s later by *Swift*-BAT [88]. The event was also detected by the *Fermi*-LAT between *T*<sup>0</sup> and *T*<sup>0</sup> + 700 s with a maximum photon energy of 5 GeV at *T*<sup>0</sup> + 142.4 s [89]. With a redshift of *z* = 0.653 and an equivalent isotropic energy release of <sup>∼</sup><sup>6</sup> <sup>×</sup> <sup>10</sup><sup>53</sup> erg in the 50–300 keV band, this event is one of the brightest GRBs ever detected by *Fermi*-LAT. The light curves show quite a conventional power-law behavior in both the X and the optical band with a temporal flux decay of index *α*XRT = 1.29 ± 0.01 and *α*optical = 1.24 ± 0.02. In the HE band, the flux followed a slightly steeper trend with *α*LAT = 0.99 ± 0.04, about 1*σ* from the mean value of the distribution of the decay indices of long GRBs detected by *Fermi*-LAT [1]. Observation by H.E.S.S. started at *T*<sup>0</sup> + 10 h, and the source was detected at ∼5*σ* level. The detection of a VHE emission at such late times into the afterglow phase implied the presence of very energetic particles accelerated (likely) at the forward shock. Similar to in the case of GRB 190114C, in [1] an SSC-emitting scenario was found to be reasonably in agreement with the observational data, although the marginal level of the significance did not allow more detailed investigation with high statistic and time-resolved spectra and light curve.
