An extremely energetic cosmic ray – an extragalactic particle with an energy exceeding ~240 exa-electron volts (EeV) – has been detected by the Telescope Array experiment’s surface detector, researchers report. However, according to the findings, its arrival direction shows no obvious source. Ultrahigh-energy cosmic rays (UHECRs) are subatomic charged particles from space with energies greater than 1 EeV – roughly a million times as high as the energy reached by human-made particle accelerators. Although low-energy cosmic rays primarily emanate from the sun, the origins of rarer UHECRs are thought to be related to the most energetic phenomena in the Universe, such as those involving black holes, gamma-ray bursts, and active galactic nuclei. Yet much about the physics and acceleration mechanisms of these particles remains unknown. Because arrivals of the most energetic UHECRs are so infrequent – estimated to be less than one particle per century per square kilometer – their detection requires instruments with large collecting areas. Here, members of the Telescope Array Collaboration report the detection of an extremely energetic cosmic ray observed by the Telescope Array (TA) experiment, a surface cosmic-ray detector array located in Utah, USA, that has an effective detection of 700 square kilometers. According to the findings, the unusually high-energy cosmic ray arrived on 27 May 2021 and had a calculated energy of about 244 EeV. Given the particle’s exceptionally high energy, the authors note that it should only experience relatively minor deflections by foreground magnetic fields, and thus, its arrival direction should be expected to be more closely correlated to its source. However, the findings show that its arrival direction shows no obvious source galaxy, or any other known astronomical objects thought to be potential sources of UHECRs. Instead, its arrival direction points back to void in the large-scale structure of the Universe – a region where very few galaxies reside. The authors suggest that this could indicate a much larger magnetic deflection than is predicted by galactic magnetic field models, an unidentified source in the local extragalactic neighborhood, or an incomplete understanding of the associated high-energy particle physics.
An extremely energetic cosmic ray observed by a surface detector array
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