Artist’s illustration of the extremely energetic cosmic ray observed by a surface detector array of the Telescope Array experiment, named “Amaterasu particle.” Osaka Metropolitan University/L-INSIGHT, Kyoto University/Ryuunosuke Takeshige

Key takeaways

  • Astronomers discovered an ultra-high-energy cosmic ray named “Amaterasu” that likely originated from outside our Milky Way galaxy.
  • These cosmic rays are incredibly rare and pack energies far surpassing human-made particle accelerators.
  • The Telescope Array observatory in Utah detected “Amaterasu” using a vast network of surface detectors spread across 700 square kilometers.
  • The origin of such high-energy particles remains a mystery, possibly linked to exotic cosmic events like black holes or gamma-ray bursts.
  • Future expansions of the Telescope Array could shed light on these cosmic enigmas by studying more particle showers across a larger area.

Space scientists investigating the mysterious origins of intense cosmic rays discovered an exceedingly unusual, ultra-high-energy particle that they believe got to Earth from outside the Milky Way galaxy.

The intensity of this subatomic particle, which is undetectable to the human eye, is similar to dropping a brick on your toe from waist height, according to the authors of a new study published Thursday in the journal Science. According to the study, it surpasses the most powerful cosmic ray ever measured, the “Oh-My-God” particle discovered in 1991.

Cosmic rays are charged particles that continually shower down on Earth from space. Low-energy cosmic rays can come from the sun, but exceptionally high-energy ones are rare. They are thought to come to Earth from distant galaxies or 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 go out to these extremely high-energy (cosmic rays), it’s around one per square kilometer each century. It’ll never pass past your hand.”

A scintillator detector (one of 507 in the original array) at its new home in the desert about the size of a ping-pong or pool table. Courtesy University of Utah

Despite years of investigation, the specific sources of these high-energy particles are still unknown. They are considered to be linked to the universe’s most energetic occurrences, such as black holes, gamma-ray bursts, and active galactic nuclei, although the largest identified so far appear to originate in voids or empty space, where no violent celestial events have occurred.

Tracking high-energy cosmic rays

The Amaterasu particle, named after the sun goddess in Japanese mythology, was found by the Telescope Array, a cosmic ray observatory in Utah’s West Desert.

The Telescope Array, which began operations in 2008, consists of 507 ping-pong table-sized surface detectors spanning 700 square kilometers (270 square miles).
According to the study, it has witnessed more than 30 ultra-high-energy cosmic rays, the largest of which was the Amaterasu particle, which impacted the atmosphere over Utah on May 27, 2021, showering secondary particles to the earth and being detected by the detectors.

“You can look …(at) how many particles hit each detector and that tells you what the energy of the primary cosmic ray was,” Mr. Matthews explained.

The event activated 23 surface detectors, with a computed energy of around 244 exa-electron volts. More than 30 years ago, the “Oh My God particle” was identified at 320 exa-electron volts.

For comparison, one exa-electron volt equals one billion gigaelectron-volts, and one gigaelectron volt = one billion electron volts. This would provide the Amaterasu particle 244,000,000,000,000,000,000 electron volts. NASA reports that an electron in the northern aurora has an average energy of 40,000 electron volts.

Extended time exposure of Telescope Station at Middle Drum with stars swirling overhead. Courtesy University of Utah

Glennys Farrar, a physics professor at New York University, noted that an ultra-high-energy cosmic ray has tens of millions of times more energy than any human-made particle accelerator, including the Large Hadron Collider, the most powerful accelerator ever created.

“What is required is a location with extremely strong magnetic fields, similar to a super-sized LHC, but natural. And the circumstances necessary are truly unusual, so the sources are quite uncommon, and the particles are dispersed over the wide cosmos, so the odds of one striking Earth are extremely slim,” added Farrar, who was not involved in the study, via email.

The atmosphere largely protects humans from any harmful effects from the particles, though cosmic rays sometimes cause computer glitches. The particles, and space radiation more broadly, pose a greater risk to astronauts, with the potential to cause structural damage to DNA and altering many cellular processes, according to NASA,.

Mysterious source

Scientists are baffled as to where these ultra-high-energy particles come from.

Matthews, a co-spokesman for the Telescope Array Collaboration, stated that the two largest recorded cosmic rays were “sort of random” — when their paths were traced back, there appears to be nothing high-energy enough to create such particles. The Amaterasu particle, in particular, appeared to originate from the Local Void, a vast expanse of empty space that borders the Milky Way galaxy.

“If you take the two highest-energy occurrences — the one we just discovered, the ‘Oh-My-God’ particle — they don’t even appear to point to anything. It should be something somewhat close. “Astronomers with visible telescopes can’t see anything really big or violent,” Matthews explained.

“It originates from a location that resembles a local empty space. It is a vacuum. So, what the heck is going on?

An extension of the Telescope Array might give some answers. According to a release from the University of Utah, once finished, 500 additional detectors would enable the Telescope Array to collect cosmic ray-induced particle showers spanning 2,900 square kilometers (approximately 1,120 square miles), an area almost the size of Rhode Island.

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