In a ground-breaking discovery, the Telescope Array Collaboration has detected an extremely energetic particle, named "Amaterasu" after the Japanese celestial sun goddess. This cosmic rays event surpasses the energy achieved by artificial particle accelerators by more than a million times. The origins of such high-energy particle remain mysterious, as tracing back the arrival direction does not reveal an obvious source, for example a galaxy.
The Telescope Array experiment, located in Utah, USA, played a crucial role in capturing this rare event. The experiment consists of a surface detector array with 507 stations and three fluorescence detectors to measure extensive air showers produced by cosmic-ray interactions with the Earth's atmosphere. The detected cosmic ray on May 27, 2021, exhibited a nearly-unprecedented energy, approximately 40 million times higher than protons produced by the Large Hadron Collider at CERN, is described in a recent issue of the journal Science, and is without exaggeration groundbreaking.
“Such energetic particles only come once per square kilometre every thousand years, so we are very lucky that we could detect it,” explains Federico Urban, who has been part of the collaboration for more than ten years. Comparisons with previous high-energy cosmic-ray events, such as those in 1991, 1993, and 2001, highlight the significance of the Amaterasu event, as it only comes second in energy to the Oh-My-God particle of 1991. The arrival direction of Amaterasu, not far from the Milky Way, suggests that there is a strong potential influence of the Galactic magnetic field, posing challenges in pinpointing the exact source.
“When you look in the sky in the direction where Amaterasu came from, there is nothing there. The explanation that the trajectory of the particle, even if it is a very heavy nucleus, could change with a strong Galactic magnetic field does not quite fit. Our different models show it really came from the middle of nowhere; there is no way that the particle could come from, say, M87, one of the biggest possible particle sources. So, it is a kind of a mystery,” explains Federico Urban, a specialist on magnetic fields in the Galaxy and outside, who is among the co-authors of the Science article.
The analysis of this event has raised intriguing questions about the possible sources of such ultra-high-energy cosmic rays (UHECRs). The researchers propose several explanations, including stronger-than-expected foreground magnetic fields or the existence of unknown particle physics at high energies.
“The issue is that the model for the magnetic field, which is telling us where to look, might be wrong, or there could be magnetic fields outside our Galaxy which are stronger than we think. And if they are there, this is very interesting, because we do more or less understand how magnetic fields form in galaxies, but if there is almost nothing, creating magnetic fields outside galaxy clusters is very difficult, there is no reason for them,” said Federico, who has developed methods to test the idea that there could be magnetic fields in the empty regions of the Universe.
The detection of Amaterasu opens new avenues for research into the most energetic phenomena in the Universe, offering insights into the acceleration mechanisms of UHECRs. As scientists grapple with the mysteries surrounding these particles, the Amaterasu event marks a milestone in our quest to understand the cosmos at its highest energy levels.