Incredibly powerful ‘cosmic ray’ signal spotted in distant space – and scientists have no clue what it is

The ‘Amaterasu particle’ is the second most powerful particle of its kind ever seen – but researchers do not even really know what it is

Andrew Griffin
Thursday 23 November 2023 19:51 GMT

Scientists have detected the second most powerful cosmic ray ever seen.

An particle charged with so much energy that it appears to defy explanation appears to have been thrown from a seemingly dark region on the edge of our galaxy and landed on Earth, astronomers say.

The astonishingly powerful charged particle is almost entirely mysterious, and should not be able to exist according to current understandings of physics. Researchers do not know where it came from, how it got here, or even what it really is.

It appears to have originated from a dark region of the Milky Way. There is no clear object in the direction from which the particle arrived, researchers say.

The mystery “suggest[s] possibilities of unknown astronomical phenomena and novel physical origins beyond the Standard Model”, said Toshihiro Fujii, from Osaka Metropolitan University, who led the work. Professor Fujii said that he believed there had been a mistake when the signal was first detected, since it showed an energy level that has not been seen in 30 years.

Dubbed the ‘Amaterasu particle’, the cosmic ray has a power comparable only to the most powerful energy cosmic ray ever seen, dubbed the ‘Oh-My-God’ particle and spotted in 1991.

The detected cosmic ray had an estimated energy of 244 EeV, comparable to the most energetic cosmic ray ever observed

The new cosmic ray was detected by the Telescope Array experiment, which brings together 507 different stations in a grid of in the Utah desert to detect cosmic rays and other phenomena. It has seen more than 30 of those ultra-high-energy cosmic rays, but none have been anywhere near the power of the ‘Oh-My-God’ particle, until now.

In 2021, scientists picked up the new cosmic ray, named Amaterasu, when it triggered 23 of those detectors. The array showed that it was coming from the Local Void, an empty part of space on the edge of our Milky Way galaxy.

“The particles are so high energy, they shouldn’t be affected by galactic and extra-galactic magnetic fields. You should be able to point to where they come from in the sky,” said John Matthews, Telescope Array co-spokesperson at the University of Utah, and co-author of the study.

“But in the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there’s nothing high energy enough to have produced it. That’s the mystery of this—what the heck is going on?”

The scientists involved in the work attempt to explain the phenomenon in a new study, ‘An extremely energetic cosmic ray observed by a surface detector array’, published today in the journal Science. But they have largely struggled to do so and the origin of the cosmic ray has defied explanation.

“These events seem like they’re coming from completely different places in the sky. It’s not like there’s one mysterious source,” said John Belz, another scientist at the University of Utah and co-author of the study. “It could be defects in the structure of spacetime, colliding cosmic strings.

“I mean, I’m just spit-balling crazy ideas that people are coming up with because there’s not a conventional explanation.”

Cosmic rays are charged particles that are thrown through the universe, almost at the speed of light, and they are almost constantly raining down on Earth. They hit the atmosphere and break apart, showering particles down onto the surface of the Earth.

But some of those cosmic rays stand out. They shoot through the universe with huge amounts of energy, which lets them avoid being pulled around by the electromagnetic fields of space.

Scientists don’t know what kind of powerful celestial event would be able to charge a particle to that degree. The objects thought of as among the most intense and extreme in the universe – such as a supernova – have nowhere near the requisite power to accelerate a particle that much, scientists say.

Researchers hope that the mystery will be cleared up in the years to come, as the Telescope Array gets expanded and becomes more sensitive. That could allow for more detailed and sensitive investigation of such particles, scientists say.

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