**4. GRB Observation at VHE: The Story so Far**

The field of VHE transient astronomy has been rapidly evolving for the last 30 years, mainly (but not only) due to the development of the imaging atmospheric Cherenkov technique. Towards the end of the last century, the first IACT experiments were built and

started operation, proving the robustness and reliability of this detection technique through the first detection of the standard candle VHE emitter, the Crab Nebula [53]. In parallel with the confirmation of the IACT technique, GRBs science was entering for the first time in a phase of systematic population studies thanks to the BATSE and EGRET instruments on board the CGRO (see Section 3) but also to the BeppoSax satellites, launched in 1996 [54]. The latter, thanks to the contemporary presence on the same platform of both wide- and narrow-field instruments, was able to provide, for the first time, arcminute localizations of GRB positions although with ∼ hours delay timescale. As reported in the previous section, the discovery of a delayed and persistent HE emission component in some of the EGRET-detected events (see, e.g., [37]) definitively pushed the search for a component also at VHE. Real-time triggers provided through the BATSE Coordinates Distribution Network (BACODINE) and the third Interplanetary Network IPN-[55], although with relative large uncertainties in the localization (&few degrees), allowed, for the first time, the rapid follow-up by ground-based telescopes including the earliest VHE facilities, such as the first IACTs and EAS arrays. Although not presenting imaging capabilities, EAS arrays were able to cover a wide portion of sky, allowing for offline search of a coincidence signal in the ultra-high-energy (UHE) gamma-ray band (&100 TeV). Such a search for emission of TeV/PeV gamma rays associated with GRBs has been extensively reported in literature by many different EAS collaborations, such as CYGNUS-I [56], HEGRA-AIROBICC [57], CASA-MIA [58], and EAS-TOP [59]. None of these revealed any convincing evidence for emission in the >100 TeV band. It is important to remark that at the time of these observations, a firm determination of GRB distance was still missing and the detection of &100 TeV photons represented a concrete possibility and an important insight into the origin (cosmological or local) of these events. A largely discussed, although not conclusive, hint for an emission in the ∼TeV band came from the Milagrito experiment [60]. Milagrito was a TeV EAS array based on the water Cherenkov detection technique, a prototype of the larger Milagro detector. The array operated between February 1997 and May 1998 in the 500 GeV–20 TeV energy range, observing 54 BATSE GRBs localized in its field of view. A possible ∼3.5*σ* evidence of TeV emission was found in the case of GRB 970417A, likely caused by photons of &650 GeV [61]. This measurement could indicate the first detection of a GRB in VHE regime; however, the weakness of the signal did not allow any spectral analysis of the event. Moreover, no other similar detection was observed by the later Milagro experiment in the same energy range, making the reliability of this observation less constraining.

The first follow-ups by an IACT at lower energies compared to EAS arrays (above ∼250 GeV), took place at the beginning of the 1990s, thanks to the Whipple 10 m reflector. These observations represented the first use of the IACT technique in exploring the GRB phenomenon complementing, although not yet overlapping, the band coverage guaranteed by the contemporaneous space-based instrumentation. Whipple reported no significant emission in the VHE band from a sample of nine GRBs observed between May 1994 and December 1995. The obtained upper limits are of the order of that expected for prompt emission if the burst emission extends to TeV energies with a band-like extrapolation without breaks or cutoff [62]. This confirmed the effectiveness of the IACT technique in proving GRB physics while pointing out some of the main difficulties of these follow-ups. Differently from EAS array, being (relatively) narrow field instruments, IACTs need to be repointed to GRB coordinates in order to start the follow-up. This introduced a delay that, for these earliest observations, ranged from 2 to 56 min. Furthermore, due to the large uncertainty in the BATSE localization of the events, the majority of the observations were performed with the source located off-axis (or in some case outside the telescope's field of view), significantly decreasing the sensitivity of the instrument and requiring multiple pointings to scan the burst region (Figure 4). The necessity of having rapid repointing and follow-up observations was the core issue in 2004 of the launch of the *Swift* satellite [63]. *Swift* operates as a multi-band satellite incorporating three different instruments: a large FoV soft-gamma detector for GRBs trigger (BAT, burst alert telescope: 15–150 keV) and

two telescopes in the X-ray (XRT, X-ray telescope: 0.3–10 keV) and UV band (UVOT, UV optical telescope) for the low-energy follow-up. These instruments were mounted on an autonomously slewing spacecraft that, using the same driven-logic of BeppoSax, made possible the observation and the precise localization of GRBs within tens of seconds from the event onset. These key features significantly improved the understanding of the early afterglow phase and its connection with the prompt emission [64]. Almost in parallel to the launch of *Swift*, the new generation of IACTs MAGIC (https://magic.mpp. mpg.de/), H.E.S.S. (https://www.mpi-hd.mpg.de/hfm/HESS/), and VERITAS (https: //veritas.sao.arizona.edu/) (all websites accessed on 10 April 2022) started operations opening a new phase in GRB study at VHE. Some of these telescopes were explicitly designed to optimize the follow-up observation of GRBs, with the aim to reach the few tens of GeV energy threshold, bridging the observational energy gap between the space-based instrumentation and enlarging the available gamma-ray horizon, one of the critical aspects for high redshift sources such as GRBs. Extensive follow-up campaigns on GRBs were performed by all IACTs collaborations along approximately 15 years of observations and they also progressively bridged the energy coverage gap with AGILE and *Fermi*. However, these extended observations did not report any conclusive evidence of VHE emission from the observed events. We briefly summarize the main outcomes of this first decade of observation.

**Figure 4.** Excess sky map for two pointing positions during the follow-up of two GRBs by Whipple telescope in 1995. The refined position of the event is marked as B\* in the plot while the IPN confidence areas are derived from BATSE and *Ulysses* data. Reprinted with permission from Ref. [62].

MAGIC (Major Atmospheric Gamma Imaging Cherenkov) is a system of two 17 m IACTs, with a ∼3.5◦ field of view located on the Canary Island of La Palma. Observations started in 2004 with a single standalone telescope until a second one was added in 2009, improving angular resolution and sensitivity. Extensive follow-up campaigns on GRBs were performed since the beginning of the operations, taking advantage of the instruments low-energy threshold (.50 GeV) combined with a very fast respositioning speed (∼7 ◦/s). Despite the continuous improvement in instrument's reaction to external GRB triggers and in data analysis along the years, no significance evidence of VHE emission was reported during the first ∼15 years of observations. However, remarkable results were achieved in terms of performance, such as the first follow-up of GRBs during the prompt emission phase for a bunch of events such as GRB 050713A (Figure 5, left panel), GRB 131030A, GRB 141026A, and GRB 150428B [4,65–67]. Furthermore, within the framework of relativistic shock-wave models, possible emission in the VHE band by synchrotron-self-Compton mechanism in afterglow has been modeled and discussed by the MAGIC collaboration in relation to the obtained upper limits on a few interesting events such as GRB 0804030 [68] and GRB 090102 (Figure 5, right panel), one of the first GRBs with simultaneous data taken with *Fermi*-LAT [5]. Although not particularly constraining, these results showed that IACT performances were mature enough to play an important role in GRB studies.

The High-Energy Stereoscopic System (H.E.S.S.) is an array of IACTs operating in Namibia since 2004. The so-called phase-I included four 12 m diameter telescopes, with an energy threshold of ∼100 GeV at zenith, and a 5 ◦ field of view. In 2012, a large 28 m diameter telescope was added to the array. This telescope is characterized by a faster repointing and large collection area (∼<sup>600</sup> <sup>m</sup><sup>2</sup> ) that guarantee an energy threshold of 50 GeV. Thus, it is a transient-oriented instrument. The introduction of the new telescope marked the beginning of the H.E.S.S. phase-II operations. Despite these improvements, also for H.E.S.S., the first 15 years of observations did not reveal any significant emission for the observed events. Collection of follow -ups and possible interpretation of the obtained upper limits are summarized in different collaboration works, such as in [69–72].

VERITAS (Very Energetic Radiation Imaging Telescope Array System) is an array of four 12 m IACTs located in Arizona operating in the &100 GeV band. The system is the successor of Whipple and has activated a GRB observing program since the beginning of the operations in 2007. VERITAS did not report any detectable VHE emission from the sample of the observed GRBs; however, in 2013, VERITAS was the only IACT able to follow up GRB 130427A, the first GRB observed at VHE (see Section 3). Unfortunately, VERITAS was only able to perform observations on GRB 130427A approximately 20 h after the event's onset. Although at that time *Fermi*-LAT was still able to detect activity in the HE band, VERITAS did not report a significant emission in the VHE range.The achieved upper limits at ∼100 GeV were able to significantly constrain the proposed emission model, pointing out tensions within the Klein–Nishina and Thomson emission regimes [73] (Figure 6).

**Figure 5.** (**Left**) MAGIC excess event rate above the energy threshold of 175 GeV compared with the *Swift*-BAT light curve for GRB 050713A, the first prompt emission followed by an IACT. The vertical line shows the beginning of observations with the MAGIC telescope. Reprinted with permission from Ref. [65]. (**Right**) MAGIC and *Fermi*-LAT overlapping upper limits for GRB 090102. These results are compared to a leptonic synchrotron+SSC afterglow model. From model and data analysis described in [5].

**Figure 6.** Combined *Fermi*-LAT spectrum including 1*σ* confidence interval and VERITAS upper limits for the late afterglow of GRB 130427A. Reprinted with permission from Ref. [73].

In parallel to these IACT observations, new EAS facilities, such as ARGO-YBJ, also started taking data in 2004 in the GeV band. No significant VHE emission was reported from any of the events located in the instrument field of view (see, e.g., [74]).

The VHE landscape on GRB study changed dramatically between 2018 and 2019 when the first detections were finally reported by the MAGIC and H.E.S.S. collaborations. These events are described in the following sections and their main parameters are summarized in Table 1.

**Table 1.** Summary of the main properties of the GRBs detected in the VHE range. GRB 201015A is included due to the strong evidence reported in [75] and is described in Section 4.4. The isotropic energy *E*iso is calculated in the 50–300 keV range for GRB 180720B, 1–10<sup>4</sup> keV for GRB 190114C, and 10–10<sup>3</sup> keV for GRB 190829A and GRB 201216C. The spectral index *α*obs is reported for the observed time-integrated spectrum (after absorption due to the EBL) over the whole observation window assuming a power-law model.

