**5. The New TeV Spectral Window: Discussion**

After decades of searches, MAGIC and H.E.S.S. observations have unequivocally proven that (long) GRBs can be accompanied by a significant amount of TeV emission during the afterglow phase. Table 4 summarizes the main properties of the GRBs detected by IACTs, and presented in detail in the previous section. The list also includes two events (namely GRB 160821B and GRB 201015A), where only a hint of excess (i.e., with a significance at ∼3–4 *σ*) was found. For the other four events, namely GRB 180720B, GRB 190114C, GRB 190829A and GRB 201216C, the detections are robust (>5*σ*). The table lists several properties, such as duration *T*<sup>90</sup> and total emitted energy *Eγ*,*iso* of the prompt emission, redshift, and information on the IACT detection (the starting time *Tdelay* of observations elapsed since the trigger time *T*0, the energy range where photons have been detected, the name of the telescope and the significance of the excess). GRB 160821B is the only one belonging to the short class; the other five being long GRBs.

In this section, we address the question why these GRBs have been detected, whether they have peculiar properties and whether they show some common behaviors that may be at the basis of the production of TeV radiation. To do that, one should be careful, since these GRBs have been followed-up under very different observational conditions and with very different time delays after the trigger time, and they span quite a large range of redshifts (from 0.078 to 1.1). Keeping in mind these differences, which have a strong impact on the detection capabilities of IACTs, we compare the observed and intrinsic properties of the population of GRBs at VHE, highlighting their similarities and differences, and discuss how they compare to the whole population.

**Table 4.** List of the GRBs observed by IACTs with a firm detection (significance > 5*σ*) or a hint of detection (3–4*σ*) above 100 GeV. The *T*<sup>90</sup> and *Eγ*,*iso* refer to the duration and total emitted energy of the prompt emission; the redshift is listed in column 3; *Tdelay* is the time delay between the trigger time *T*<sup>0</sup> and the time when IACT observations started; *Erange* defines the energy range of the detected photons. The name of the telescope which made the observation and the significance of the detection are listed in the last column.

