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Scientists studying space are trying to comprehend the puzzling origins of intense cosmic rays. They have identified an extremely rare, ultra-high-energy particle that they believe journeyed to Earth from beyond the Milky Way galaxy.
This subatomic particle’s energy, although invisible to the naked eye, is equivalent to dropping a brick on your toe from waist height, as per the authors of new research published in the journal Science. The study reveals that this particle rivals the most energetic cosmic ray ever detected, which was observed in 1991.
Cosmic rays, charged particles that travel through space and reach Earth continuously, are present everywhere. While low-energy cosmic rays are often from the sun, extremely high-energy ones are exceptional and believed to originate from other galaxies and extragalactic sources.
“If you hold out your hand, one (cosmic ray) goes through the palm of your hand every second, but those are really low-energy things,” said study coauthor, John Matthews, a research professor at the University of Utah. “When you get out to these really high-energy (cosmic rays), it’s more like one per square kilometer per century. It’s never going through your hand.”
Despite extensive research, the precise origins of these high-energy particles remain elusive. They are linked to the most energetically powerful phenomena in the universe, such as those involving black holes, gamma-ray bursts, and active galactic nuclei. However, the most significant ones discovered so far seem to come from empty space devoid of large celestial events.
Tracking high-energy cosmic rays
The recently identified particle, dubbed the Amaterasu particle after the sun goddess in Japanese mythology, was detected by a cosmic ray observatory in Utah’s West Desert known as the Telescope Array.
Operating since 2008, the 507 ping-pong table-size surface detectors that make up the Telescope Array cover 270 square miles. They have observed over 30 ultra-high-energy cosmic rays, but none larger than the Amaterasu particle. It struck the atmosphere above Utah on May 27, 2021, causing secondary particles to fall to the ground where they were picked up by the detectors, according to the study.
“You can look …(at) how many particles hit each detector, and that tells you what the energy of the primary cosmic ray was,” Matthews explained.
The event triggered 23 of the surface detectors, with a calculated energy of about 244 exa-electron volts. The most energetic cosmic ray ever observed, detected over 30 years ago, was 320 exa-electron volts.
To provide a reference, 1 exa-electron volt equals 1 billion gigaelectron-volts, and 1 gigaelectron volt is 1 billion electron volts. That would make the Amaterasu particle 244,000,000,000,000,000,000 electron volts. By comparison, the typical energy of an electron in the polar aurora is 40,000 electron volts, according to NASA.
An ultra-high-energy cosmic ray carries tens of millions of times more energy than any human-made particle accelerator such as the Large Hadron Collider, the most powerful accelerator ever built, explained Glennys Farrar, a professor of physics at New York University.
“What is required is a region of very high magnetic fields — like a super-sized LHC, but natural. And the conditions required are really exceptional, so the sources are very very rare, and the particles are dissipated into the vast universe, so the chances of one hitting Earth are tiny,” said Farrar, who wasn’t involved in the study, via email.
The Earth’s atmosphere largely shields humans from any harmful effects of cosmic rays, although they sometimes cause computer glitches. However, the particles and space radiation more broadly pose a greater threat to astronauts, potentially causing DNA damage and interfering with cellular processes, according to NASA.
Mysterious source
The source of these ultra-high-energy particles continues to puzzle scientists.
Matthews, a co-spokesman for the Telescope Array Collaboration, said the two most significant recorded cosmic rays seemed “sort of random” when their trajectories were traced back. In particular, the Amaterasu particle appeared to originate from what is known as the Local Void, an empty area of space bordering the Milky Way galaxy.
“If you take the two highest-energy events — the one that we just found, the (1991) particle — those don’t even seem to point to anything. It should be something relatively close. Astronomers with visible telescopes can’t see anything really big and really violent,” Matthews said. “It comes from a region that looks like a local empty space. It’s a void. So what the heck’s going on?”
An expansion of the Telescope Array may provide some answers. Once completed, 500 new detectors will enable the Telescope Array to capture cosmic ray-induced particle showers across 1,120 square miles — an area nearly the size of Rhode Island, according to the University of Utah statement.
