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| Illustration of an ultra-high-energy-cosmic ray hitting Earth (Credit: Osaka Metropolitan University /L-INSIGHT, Kyoto University/ Ryuunosuke Takeshige) |
Astronomers have detected the second most powerful cosmic ray ever recorded, but it seems to have come from an empty region of the universe known as a cosmic void, where there is nothing obvious it could have originated from.
This mysterious cosmic ray, which is probably a charged particle nucleus of some variety, such as carbon or oxygen, has been dubbed Amaterasu after the Japanese sun goddess.
Possible explanations for it include magnetic fields steering the particle off-course, an invisible source in another galaxy or exotic new cosmic phenomena.
“The first time I saw the result, I thought that it should be some mistake, it was much more energetic than my expectations,” says Toshihiro Fujii at Osaka Metropolitan University in Japan. “At the beginning, I thought it was a bit unlucky as it had no source, but actually it is lucky, because we found a new mystery.”
Fujii and his colleagues spotted the event using the Telescope Array in Utah, a collection of more than 500 sensors that can detect showers of particles raining down from extremely energetic particle collisions with Earth’s atmosphere.
By working back from the sensors’ detections and using particle physics models, researchers can reconstruct the energy and direction of an incoming particle.
On 27 May 2021, a group of the sensors lit up with signals that implied a particle had exploded above them with an energy of about 244 exaelectronvolts (EeV), which is equivalent to the energy of a tennis ball moving at nearly 100 kilometres per hour, squeezed into an atomic-sized object.
Events at this energy are extremely rare, and only four particles have been recorded above 200 EeV. The most energetic particle, a 320 EeV particle called the “Oh-My-God particle”, still isn’t fully explained.
These ultra-high-energy cosmic rays are thought to be produced in exotic astrophysical events, such as in supermassive black holes or gamma ray bursts. But when Fujii and his colleagues examined the part of the sky that the particle had come from, they could see no obvious source.
It is possible that the cosmic ray came from somewhere else and veered off course because of the effect of an object’s magnetic fields, but with the particle still having such a high energy when it arrived, this change of direction can only have been minimal, unless our models of extragalactic magnetic fields are incorrect, says Fujii.
The higher the charge of the particle nucleus, the bigger the deflection, but even the maximum deflection, which would come from a highly charged iron nucleus, doesn’t provide a change in trajectory that leads to a possible source.
Higher-energy particles lose energy when interacting with the cosmic microwave background (CMB), a background buzz of radiation left over from the big bang. This limits how far these ultra-energetic rays can travel, but if the charged particle is a more exotic phenomenon that doesn’t interact with the CMB, then it could have come from much more distant galaxies that we can’t detect, says Fujii.
It is a puzzling scenario, says Justin Bray at the University of Manchester, UK, partly because we have so few examples of extremely high-energy cosmic rays and many of them have been detected in different ways, which makes it difficult to compare and analyse them as a group.
Although differentiating between possible sources for Amaterasu is tricky, says Bray, claims of a new particle would require extraordinary evidence. “It is a fairly out-there possibility, but of course very exciting if true.”
Journal reference
Science DOI: 10.1126/science.abo5095
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