**1. Introduction**

In 2019, the announcement of the first detection of VHE gamma-ray emission from GRB 180720B [1], GRB 190114C [2], and GRB 190829A [3] represented a long-awaited result for the astrophysical community and the end of a quest lasting for more than twenty years. The detection of a VHE counterpart of GRBs always posed a major challenge for imaging atmospheric Cherenkov telescopes (IACTs) from both the technical and the scientific point of view see, e.g., [4,5]. On the other hand, catching such a signal has a crucial impact on understanding the poorly-known physics of these objects during the different phases of their emission, motivating the continuous efforts in the VHE observational window. In fact, the observed radiation still has an uncertain origin in many aspects. According to the widely accepted relativistic shock model originally proposed in [6], GRB emission arises from the conversion of the kinetic energy of a relativistic outflow into electromagnetic emission. The details of this conversion remain poorly understood. However, the dissipation might happen in the form of collisionless shocks between the relativistic flow itself (internal shocks, responsible for the prompt phase) or with the circumburst medium (external shocks, responsible for the afterglow emission phase). Alternatively, other dissipation mechanisms have been considered in literature and, noticeably, the possibility of having magnetic reconnection events as the base for particle acceleration; see, e.g., [7,8]. The nature of the possible radiative processes at work is also not firmly established yet. Particles inside the outflow and accelerated towards relativistic regime can emit the observed highenergy photons via many possible non-thermal mechanisms, in particular during the early

**Citation:** Berti, A.; Carosi, A. The Detection of GRBs at VHE: A Challenge Lasting for More than Two Decades, What is Next? *Galaxies* **2022**, *10*, 67. https://doi.org/ 10.3390/galaxies10030067

Academic Editors: Elena Moretti, Francesco Longo and Yi-Zhong Fan

Received: 10 March 2022 Accepted: 18 April 2022 Published: 10 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

afterglow phase. In this regard, the prompt-to-early-afterglow phase still remains the least understood in GRB dynamics. Prompt emission spectra have been largely fitted through the so-called Band function [9], an empirical function composed by two smoothly connected power-law functions at a specific break energy. While historically the band function worked quite well in fitting prompt spectra in the 10 keV–1 MeV range for many GRBs, more recent works have showed that extra emission components in the form of an additional powerlaw and/or a photospheric blackbody component are needed to better fit the observed emission both at lower and at higher energies with respect to the GRB peak energy (see, e.g., [10,11] and references therein). Such components might account for a revision of the theoretical interpretation of the observed radiation as possible synchrotron emission from electrons accelerated within the relativistic outflow. Synchrotron emission has been shown to be in tension with experimental data in many events; see, e.g., [12,13]. Nevertheless, synchrotron is believed to play an essential role in GRB physics and it has been largely considered as the most natural process to explain the GRB sub-MeV emission both during the prompt and afterglow; see, e.g., [14–16]. Furthermore, it has also been suggested that the high-energy photons above ∼10 MeV observed by the *Fermi*-LAT (Large Area Telescope) [17], and extending after the end of the prompt emission, might be generated by synchrotron radiation produced in external shocks [18]. However, the observation of an emission component at VHE, as recently detected by current IACTs, challenges the synchrotron-alone emission models and, ultimately, the particle acceleration mechanisms at work in GRBs. In internal/external relativistic shock models, particles can be accelerated up to a maximum Lorentz factor achieved when the comoving acceleration time matches the typical radiative cooling time. The corresponding maximum photon's energy emitted by a synchrotron is around ∼50 MeV in the comoving frame corresponding to an observed *Emax*∼50 MeV × Γ/(1 + z) (synchrotron burnoff limit), where Γ is the bulk Lorentz factor of the relativistic outflow and z is the redshift of the source. In the case of GRBs, arguments for the hypothesis of an emitting region moving towards the observer with a bulk Lorentz factor Γ∼*f ew* × 100 are known and used to solve inconsistencies between the observed non-thermal emission above the pair production threshold (*γγ* → *e* +*e* −) and the time variability observed during the prompt phase; see, e.g., [19]. With the *Fermi* satellite, some firm estimations for Γ have been achieved using the maximum photon's energy detected by *Fermi*-LAT in the GeV band. For some particularly bright GRBs, values exceeding <sup>≈</sup>10<sup>3</sup> , as in the case of Γ∼900 for GRB 080916C [20] and Γ∼1200 for GRB 090510 [21], have been measured. Although those values dramatically differ from any other relativistic motion observed in other astrophysical sources, they would still appear moderate if considering the signal caught by IACT in the hundreds of GeV or even TeV band. Furthermore, after the end of the prompt phase, Γ decreases with time [22], implying that the maximum energy achievable by synchrotron photons decreases as well. Thus, HE and VHE signals detected deeper in the afterglow phase, such as in the case of GRB 190829A, abundantly exceed the synchrotron burnoff limit, challenging the simple shock acceleration/synchrotron model. The complexity of scenarios provided by the latest IACT results shows a still unsatisfactory level of comprehension of GRB physics and the importance of continuing the observation of GRBs in the VHE band with next-generation IACTs. In the coming decades, the premier facility for VHE astrophysics will be the CTA observatory that will perform observations in the >10 GeV range with unprecedented photon statistics and sensitivity, allowing to investigate the parameter space of a wide range of VHE-transient emitters and their characteristics.

In this paper, we will revise the main experimental results that historically helped in shedding light into the GRB physics in the HE (high energy, *E* & 100 MeV) and VHE domains. The paper is organized as follows: in Section 2 we briefly introduce the theoretical emission models used to interpret GRB HE and VHE emission. In Sections 3 and 4 we will summarize the main experimental steps that brought us to the detection of GRBs in the HE and VHE band. Section 5 investigates the open issues that still affect the characterization of GRBs at VHE and that will hopefully be solved by next-generation instruments finally described in Section 6.

#### **2. Models for HE and VHE Emission in GRBs**

Although not within the primary scope of this paper, it is important to briefly summarize the main interpretative models able to explain the emission at the highest energies. Many theoretical models were proposed in the last decades to explain the emission from GRBs, with predictions extending to the HE and VHE range. Usually, in these models, the origin of HE and VHE emission can take place in both internal or external shocks. In both cases, either leptonic or hadronic processes might be considered as possible explanation of the observed emission. As already mentioned in the previous section, synchrotron emission is one of the most discussed for the emissions in the keV-MeV band. At higher energy, synchrotron photons might interact through inverse Compton with ultra-relativistic electrons of the outflow. This amplifies the energy of the seed photons by a factor of *γ* 2 *e* , where *γe* is the electron's Lorentz factor. Depending on the specific microphysical parameters of the emitting region, this synchrotron self-Compton (SSC) emission can arise and easily produce photons in the HE and VHE ranges. Detailed predictions for such a model are given in [23,24], where the suppression of inverse Compton due to the Klein–Nishina (KN) effect is also widely discussed. This effect can explain the delay observed between the keV and HE emission (see Section 3) if the KN regime is dominant at early times, but then at late times the inverse Compton enters the Thomson regime; see, e.g., [25–27]. Hadronic particles can be also shock-accelerated in the same way as leptons, influencing (potentially) the HE and VHE emission. Hadronic models comprise synchrotron emission from protons or cascade emission (synchrotron) from secondary pairs [28,29]. In the synchrotron scenario, the delay between low- and high-energy emission can be explained as the time required to accelerate protons to high enough energies. However, being a poor emitter compared to leptons, proton energy is mainly lost through p-*γ* interactions rather than by synchrotron. In this case, the required energy budget to achieve comparable emission level with leptonic processes is normally well above the observed ones (&10<sup>55</sup> erg), although this requirement can be relaxed with a narrow jet opening angle (<1 ◦ ). In the case of external shocks, one of the main models considered for HE and VHE emission is SSC at the (external) forward shock. In such a case, a separate component with a second peak at high energies is expected [14,15]. This model was largely used to explain the HE emission in some *Fermi*-LAT bursts (see, e.g., [30–32]), as we will describe further in the next sections. SSC is proposed to produce HE photons also in the reverse shock [33] and it was shown to explain the HE component of some GRBs [34]. Furthermore, in the external shock scenario, hadronic models are also a possible option in order to account for HE and VHE emission. However, as in the internal origin case, hadronic processes suffer the same issue on the required energetics, although a possible non-dominant contribution to the overall HE-VHE emission cannot be completely excluded [35]. We refer to [36] for a more detailed review on theoretical emission models.
