**4. Discovery of a TeV Emission Component in GRBs**

The robust theoretical framework developed throughout the years to explain the afterglow radiation predicts that, to some extent, GRBs should be TeV emitters (Section 2). Observations in the HE band, and in particular the presence of &GeV photons with energies up to ∼100 GeV, support this possibility. On the other hand, from the observational side, the search for such emission is hampered by several drawbacks. Space-born telescopes, such as Fermi-LAT, sensitive up to few hundred GeV, have an hard time with GRBs due to their low *<sup>γ</sup>*-ray photon flux at the highest energies (∼10<sup>2</sup> GeV), caused by their cosmological distance and strong EBL absorption. These difficulties can be overcome by the much larger effective area of IACTs in the common energy range of sensitivity (50–300 GeV). As a downside, IACTs have a small field of view (a few degrees wide), higher low-energy threshold (&50–100 GeV), and reduced duty cycle (less than 10%).

In the last decades, IACTs have performed a huge effort to become instruments suitable for GRB observations. In particular, the efforts have been focused in two directions: (i) the development of fast repointing systems to promptly react to GRB alerts and start observations with delays of a few tens of seconds after the trigger time, and (ii) the extension of the energy threshold below 100 GeV, important to reduce the impact of the EBL attenuation on the detection probability of cosmological GRBs.

After a decade of VHE observations resulting in non-detections, the first announcement of GRBs detected by IACTs arrived in 2019, thanks to the MAGIC and H.E.S.S. telescopes [139]. These detections have firmly established that GRBs can be bright sources of TeV radiation. Somewhat unexpectedly, VHE emission was also detected several hours/a few days after the GRB onset, and up to energies of &3 TeV. The timescales of the detections

place the origin of the emission in the afterglow phase. The TeV emission has been studied and interpreted in a multi-wavelength context, in order to evaluate the properties and the nature of the responsible radiation mechanisms. In particular, investigations have focused on SSC, external inverse Compton (EIC), and synchrotron radiation.

In this section, all GRBs for which a detection (significance > 5*σ*) or a hint of detection (significance between 3 and 5*σ*) has been claimed by Cherenkov telescopes, are presented. These are, in total, six events (one short and five long): GRB 160821B (Section 4.1), GRB 180720B (Section 4.2), GRB 190114C (Section 4.3), GRB 190829A (Section 4.4), GRB 201015A (Section 4.5) and GRB 201216C (Section 4.6). For each event, we start with a brief description of the prompt and afterglow multi-wavelength observations. Then, we describe VHE observations and summarize the main results. These detections being a novelty, and some of them laying close to the sensitivity detection threshold of the instrument, we describe in detail the VHE data analysis, the calculation of the significance excess at the GRB position (following the usual prescription used for VHE sources presented in [140]), and the methods adopted for the derivation of the spectral energy distribution (SED) and of the light-curves. For each GRB, we also present the interpretations that have been put forward in the literature. A discussion on the main common properties and differences among this initial population of VHE GRBs and with respect to the whole GRB population is presented in Section 5, where we also address the question of what we have been learning from these few detections.

In this section, all quoted times refer to the time elapsed from the trigger time *T*<sup>0</sup> of the Swift-BAT or Fermi-GBM instrument, as will be specified. Photon indices are given in the notation *N<sup>ν</sup>* ∝ *ν α* , while temporal indices are defined by *F*(*t*) ∝ *t β T* .
