# Gamma-ray Bursts: 50 Years and Counting!
## 9 The Very High Energy Era (2019-Ongoing)  
The use of the photon-pair-conversion technique for the detection of gamma photons in space is severely limited by the volumes of the detectors. In order to observe photons with \(\geq\)TeV energies (VHE), the dimensions of the detectors are such that they make it impossible to observe this radiation from space. In order to observe photons of such energies, it is therefore necessary to use different techniques, exploiting the Earth's atmosphere as the \"medium\" of the telescope and observing the effect of the interaction of such photons with it. Gamma rays (as well as primary cosmic rays) crossing the atmosphere collide with atmospheric nuclei, producing secondary particles that, in turn, generate a cascade of particles as they traverse the atmosphere. The intricate interplay of these particles cascading down through the atmosphere creates an extensive spread of ionization and fluorescence, offering a unique signature for detection called an \"Air Shower\". There exist two types of ground observatory for VHE: Imaging Air Cherenkov Telescopes (IACTs) and Extensive Air Shower (EAS) experiments. Cherenkov experiments consist of almost-optical telescopes devoted to detecting the Cherenkov light emitted by particles produced in air showers and moving superluminally in the atmosphere. EAS experiments are huge arrays or carpets of particle detectors that directly collect the particles in air showers. Cherenkov experiments have lower energy thresholds but also a lower duty cycle, as well as a smaller field of view, when compared to EAS experiments. Since as early as 1994, various theoretical models have predicted the possibility of the existence of a very high TeV energy component of the GRB emission [171; 172; 173; 174; 175]. This stimulated a search for this component of GRB emission with various instruments, however, with poor results. Some first tentative hints of TeV emissions from GRBs came with the claims of the detection of GRB920925C by the AIRshower Observation By angle Integrating Cherenkov Counters (AIROBICC) [176] and GRB970417A by [PERSON] [177; 178]. The HEGRA AIROBICC array was an array of \(7\times 7\) 40 cm diameter PMT-based air Cherenkov integrating telescopes located in La Palma (Canary Islands) and was sensitive up to a few tens of TeV. After searching for GRBs above 20 TeV within AIROBIC's field of view, using data gathered between March 1992 and March 1993, evidence of a \(\sim\)2.7 \(\sigma\) detection from GRB920925c was reported. However, since the \"signal\" preceded the activation of WATCH (an all-sky X-ray monitor on board the GRANAT satellite [179; 180]) by \(<\)1 mm and was located approximately 9\({}^{\circ}\) away from the position identified by this instrument, no evidence was claimed. Milagrito, a prototype of the Milagr EAS experiment, consisted of a flat array of 228 photomultiplier tubes (PMTs) submerged in a light-proof water pool with a size of \(\sim\)42 \(\times\) 42 m\({}^{2}\). In 2000, Milagrito reported evidence for an emission above 650 GeV from GRB970417A, with a (post-trial) probability of 1.5 \(\times\) 10\({}^{-3}\) of being a background fluctuation. GRB970417A was a weak, soft GRB first observed by BATSE. Still, this was not enough to claim evidence of VHE emission from GRBs with full certainty. Despite enormous and growing efforts, no evidence was found, and only upper limits from various collaborations were set [181; 182; 183; 184; 185] until 2019, with the observation of GRB190114C by the Major Atmospheric Gamma-ray Imaging Florian Goebel Cherenkov Telescopes (MAGIC) [186; 187]. On 15 January 2019, MAGIC, a system of two IACTs located on the island of La Palma, reported the first observation of 0.2-1 TeV photons from GRB190114C. Triggered by the _Swift_-BAT alert, the MAGIC telescopes, thanks to their fast repointing capabilities, were able to start the observations of the GRB after just 57 s, \"revealing a distinct emission component of the afterglow with power comparable to that of the synchrotron component\" [187].