**6. Conclusions and Future Prospects**

The recent discoveries performed by the current generation of Cherenkov telescopes in the VHE band have opened a new observational spectral window on GRBs. The presence of a TeV afterglow component has been unequivocally proven and the studies on the currently available samples have demonstrated the potential that such detections have in probing several long-standing open questions in the GRB field. These first studies have focused on the identification of the responsible radiation mechanism, which is the first issue to address, and the comparison of the energetics, luminosity, and temporal behavior of the VHE component with respect to emission at lower frequencies. A modeling of multi-wavelength data covering from radio up to TeV energies was performed, giving interesting insights on the shock micro-physics conditions.

Limitations to the robust use of VHE data for afterglow modeling are imposed by the severe modification of the intrinsic spectrum cased by the energy-dependent fluxattenuation induced by EBL. GRBs with redshift *z* > 0.4, four out of six in the current VHE sample, are strongly affected by EBL absorption starting from hundreds of GeV. This implies large uncertainties on the shape and photon index of the intrinsic VHE spectrum. As a result, firm conclusions on the origin and spectral regime of the TeV component cannot be drawn yet. The low-energy extension of the range of sensitivity of IACTs is then fundamental for reaching a larger rate of detections and a more robust determination of the spectral index of the TeV component.

A full comprehension and exploitation of TeV data is expected to be reached thanks to the next generation of Cherenkov telescopes. The Cherenkov Telescope Array (CTA) will be a huge step forward for the detection of GRBs in the VHE band. The major upgrades with respect to the current generation of Cherenkov telescopes that will impact GRB observations are: (i) a lower energy threshold (.30 GeV), (ii) a larger effective area at multi-GeV energies (<sup>∼</sup> <sup>10</sup><sup>4</sup> times larger than Fermi-LAT at 30 GeV) and (iii) a rapid slewing capability (180 degrees azimuthal rotation in 20 s). Moreover, its planned mixed-size array of large, medium and small size telescopes (called LST, MST and SST, respectively) situated at two sites in the northern and southern hemispheres will provide a full sky coverage from few tens of GeV up to hundreds of TeV. CTA will have a much better sensitivity and a broader energy range with respect to current ground-based facilities. A comparison is shown in Figure 30. At the present stage, the first prototype of the LSTs has been built and is operative under a commissioning phase<sup>9</sup> in the northern site at the Roque de los Muchachos Observatory in La Palma. Despite these performance improvements, the expected CTA detection rates of GRBs will be influenced anyway by the relatively low duty cycle affecting IACTs and by the synergies with other instruments. Indeed, Cherenkov telescopes' repointing relies on external triggers coming from space satellites. Assuming that currently operating space telescopes will be still operative, GRB alerts will be mostly provided by Swift-BAT and partially by the Fermi-GBM, and in the future by the French-Chinese mission Space-based multi-band astronomical Variable Objects Monitor (SVOM [232]).

**Figure 30.** CTAO differential sensitivity<sup>10</sup> (defined as the minimum flux needed to obtain a 5 standard-deviation detection of a point-like source) for 50 h of observations with the Northern and Southern array compared to the sensitivity of several other Cherenkov telescopes and with Fermi-LAT (1 year).

BAT observes around 92 GRBs per year with a typical localization error of a few arcmin [233]. The good localization (later refined by XRT to a few arcsec) is fundamental for Cherenkov telescopes, given their limited field of view (e.g., about 4◦ for the LSTs and 7 ◦ for the MSTs). The GBM provides a much higher number of alerts, around 250 per year but with a larger localization error, from 1–3◦ up to 10◦ , which makes follow-up with IACTs very challenging. In the case of such large localization errors, CTA can exploit the so-called divergent mode for observations, which is currently under study [234]. In this pointing strategy, each telescope points to a position in the sky that is slightly offset to extend the field of view. Concerning future instruments, SVOM is expected to provide Swift-like alerts at a rate of ∼60–80 GRBs/yr with a localization error <1 ◦ , including also 10 GRBs/yr with redshift *z* < 1.

Available estimates of the CTA detection rate of GRBs are reported in [235]. These studies were performed before the discovery of TeV emission. They are based on Swift-like alerts (triggered by Swift-BAT or SVOM) and Fermi-GBM alerts. The predicted detection rate is around a few GRBs per year, depending on the energy threshold of the observation and on the observation delay [235]. An updated study that considers current knowledge of TeV emission in the afterglow of GRBs is in progress [236].

Despite for decades GRBs' hunting by Cherenkov telescopes has been primarily focused on reaching low energy thresholds in order to explore the multi-GeV band, these first detections have shown that photons above TeV energies can be produced in GRBs and can be detected. This is mostly valid only for nearby events of redshift below 0.1–0.2, where EBL attenuation is not too severe. The exploration of the GRB emission component above 1 TeV can be of potential interest for SSTs and for the ASTRI Mini-Array. The ASTRI Mini-Array, currently under construction, will be an array of nine imaging atmospheric dual-mirror Cherenkov telescopes at the Teide Observatory site, expected to deliver the first scientific results in 2023. After the detection of GRB 190114C, the capabilities of the ASTRI Mini-Array in detecting and performing spectral studies of an event similar to the MAGIC GRB have been explored [237]. GRB 190114C has been taken as a template to simulate possible GRB emission from a few seconds to hours, and has been extrapolated to 10 TeV on the bases of model predictions.

The results demonstrate that the instrument will be able to detect afterglow TeV emission from an event such as GRB 190114C up to ∼ 200 s (see the comparison between the GRB observed flux at 1 TeV and the differential ASTRI Mini-Array sensitivity in Figure 31). By moving the GRB at a smaller redshift (down to *z* = 0.078, the redshift of the TeV GRB 190829A), the time for which the GRB is detectable increases up to <sup>∼</sup>10<sup>5</sup> (however, the light-curve in this case should be re-scaled by the lower energetics of nearby events). Nearby GRBs are then potential target of interests for the ASTRI Mini-Array. These are certainly rare events, but their detection will provide a wealth of information, with spectra that can be characterized up to several TeV [237].

In conclusion, after decades of huge efforts, current ground-based VHE facilities have started a new era in the comprehension and study of GRB physics. Their breakthrough detections allow unprecedented studies. As discussed in this review, many open questions in afterglow physics can largely benefit from the inclusion of TeV data. The first detections are providing glimpses of such a huge potential. Luckily, we are at the dawn of the VHE era thanks to the upcoming CTA observatory, which will assure major upgrades in sensitivity, energy range, temporal resolution, and sky coverage. Future observations, if complemented by simultaneous observations in X-rays and at ∼ GeV energies, will play a paramount role to improve our knowledge on the physics of GRB during the afterglow phase and hopefully also in the prompt phase. In particular, the afterglow SSC one-zone model will be tested to understand whether it can grasp the main properties of the VHE emission or if a revision of our comprehension on the particle acceleration processes, shock micro-physics and radiation mechanisms is needed.

**Figure 31.** Light-curve of GRB 190114C at 1 TeV (dotted purple curve) compared to the sensitivity of the ASTRI Mini-Array. The yellow dashed and green dot-dashed curves show how GRB 190114C rescaled at redshift *z* = 0.25 and *z* = 0.078, respectively, would appear. From [237].

**Author Contributions:** All authors, D.M. and L.N., have contributed to writing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (https://www.swift.ac.uk/xrt\_curves/).

**Conflicts of Interest:** The authors declare no conflict of interest.
