# Gamma-ray Bursts: 50 Years and Counting!
## 10 The Record Breaker: GRB221009A, the B.O.A.T. On 19 October 2022, an unprecedented GRB event was observed by several instruments: GRB221009A. This event was so extreme that it broke the record as the brightest and most energetic GRB ever observed [197, 198, 199]. First detected by both the _Fermi_[200] and _Swift_[201] satellites, the GRB lasted around seven minutes, but the multiwavelength afterglow remained detectable for more than a month. This led to one of the biggest and most successful follow-up campaigns ever conducted, which led to the observation of the emission on an unprecedented 15 orders of magnitude on the electromagnetic spectrum, from radio emissions to VHE gamma rays. The burst completely saturated the detectors aboard _Fermi_, which captured gamma-ray photons with energies above 100 GeV [200]. GRB221009A produced the largest number of very high energy (VHE) photons ever observed by scientific instrumentation: before GRB221009A, the number of VHE photons detected in the entire history of GRB astronomy amounted to only a few hundred. For GRB221009A, however, the LHAASO alone saw more than 5000 VHE photons, some of these having a record energy of 18 TeV [196]. This claim is still debated, though. Still, photons of these energies are difficult to explain within the modern standard paradigms and prompted a new impulse in the development of theoretical models. At the moment, there is still no consensus about the mechanism for generating these highly energetic gamma photons, which are difficult to explain with the standard synchrotron- and synchrotron-self-Compton-based leptonic models for the afterglow, and many different mechanisms and jet geometries are continuously being proposed to explain them (e.g., [202, 203, 204, 205]). Some proposals have been put forward in which GRB221009A would be a huge accelerator of UHECRs, whose propagation would induce an electromagnetic cascade in the extragalactic medium. The line-of-sight component of this flow could explain the detected \(>10\) TeV emission [206, 207]. The presence of photons of such energies, coming from an estimated distance of about \(z=0.151\)[208], is at odds with theoretical models because of the expected attenuation due to the extragalactic background light (EBL). This has led to much speculation on the presence or absence of exotic effects to explain the detection, with different groups proposing different mechanisms, such as the presence of LIV effects [209, 210] or of axion-like particles (ALPs) [211; 212]. Other groups have, however, highlighted how these observations, although exceptional, can still fit into standard paradigms. Despite the extensive follow-up campaign, no multi-messenger detection occurred, managing to place stringent constraints on the emission mechanisms thanks to the non-observation of neutrino emission [213; 214; 215]. An important claim was made following an analysis of the spectral evolution of GRB221009A, as seen by _[PERSON]_/GBM, reporting the discovery of a narrow emission feature at around 10 MeV with very high significance (\(>\)6\(\sigma\)), interpreted in the paper as a blue-shifted annihilation line of relatively cold electron-positron pairs (\(k_{B}T\ll m_{e}c^{2}\)) [216]. This feature has never been observed in any other GRB, and its detection would be made possible by the exceptional brightness of the event, which dwarfs even the next-brightest bursts (such as GRB130427A). The presence of an excess around 10 MeV was also reported in the spectral analysis by the _[PERSON]_ team [200]. In [197], it is argued that, while GRB2201009A might not be the intrinsically brightest GRB ever, its proximity to Earth makes it so that it is the brightest ever observed. In the same paper, they report an estimated timescale of approximately 10,000 years for any such event to happen again.