Astronomers Detect Extremely Powerful Cosmic Ray of Mysterious Origin

Researchers have detected an extremely energetic cosmic ray from an unknown source in the universe.

The detection of a cosmic ray with such high energy is "exceedingly rare" and the latest discovery, documented in a study published in the journal Science, could shed new light on the origin of these mysterious phenomena.

Cosmic rays are charged particles that travel through space at almost the speed of light. When they arrive at Earth, they collide with gas molecules in the atmosphere, creating a shower of billions of secondary particles that scientists can detect with the right instruments.

The energy level of cosmic rays spans a very broad range. The lowest energy cosmic rays that scientists observe primarily come from our sun, which spews out electrons and other particles.

Intermediate energy cosmic rays, meanwhile, are believed to come from powerful events associated with exploding stars—known as supernovae—in our galaxy. The origins of the highest energy cosmic rays, however, remain a mystery. These particles—known as ultrahigh-energy cosmic rays (UHECRs)—were first detected in 1962.

"The nature of the most powerful particle accelerators in the universe is a 60-year-old mystery," Noemie Globus, one of the authors of the study, who is affiliated with the Institute of Physical and Chemical Research in Japan and the University of California, Santa Cruz, told Newsweek.

UHECRs have energies greater than 1 exa-electronvolt (EeV)—which is roughly one million times greater than those achieved by the most powerful particle accelerators ever made by humans.

While their origins remain unknown, a number of possible candidates have been suggested as being sources of UHECRs. These include some of the most extreme phenomena in the universe, such as those involving black holes (regions of space-time where gravity is so strong that nothing, including light, can escape them), gamma-ray bursts (immensely powerful explosions observed in distant galaxies) and active galactic nuclei (compact regions at the center of some galaxies that emit huge amounts of energy across the electromagnetic spectrum).

In addition to their mysterious origins, little is known about the physics and acceleration mechanisms of UHECRs. Studying them is challenging because they are so rarely detected. Arrivals of the most energetic UHECRs on Earth are so infrequent—rates are estimated to be less than one particle per century per square kilometer (or roughly 0.4 square miles)—that they require instruments with large detecting areas.

One such instrument is the Telescope Array (TA) experiment—a specialized cosmic ray detector array located in Utah that has an effective detecting area of around 280 square miles. Operated by an international team of scientists, the TA experiment began in 2008. Since then, the team has observed around 30 of the highest-energy cosmic rays. Most of these events are close to 100 EeV in energy—equivalent to 100,000,000,000,000,000,000 electronvolts (eV).

"Particles of this energy are exceedingly rare," John Matthews, one of the authors of the Science study with the University of Utah, told Newsweek.

But now the TA team have reported the detection of an exceptionally high-energy cosmic ray. This particle, observed on May 27, 2021, is estimated to have had a staggering energy level of 244 EeV. That is equivalent to 244,000,000,000,000,000,000 eV.

"When I first discovered this ultra-high-energy cosmic ray, I thought there must have been a mistake, as it showed an energy level unprecedented in the last three decades," Toshihiro Fujii, the lead author of the research who is affiliated with Kyoto University and Osaka Metropolitan University in Japan, said in a press release.

The energy level of the recently detected particle is so high, in fact, that it is comparable to the most energetic cosmic ray ever observed—dubbed the "Oh-My-God" particle. This particle, detected in 1991, was estimated to have an energy level of 320 EeV—equivalent to 320,000,000,000,000,000,000 eV.

"[The 2021] event is also exceptional in that it is close in energy to the Oh-My-God particle and was observed by a very different technique," Matthews said. "This shows us that despite being very rare, and so far unexplained, these particles do exist."

Fujii and colleagues decided to dub their latest discovery the "Amaterasu" particle, after a sun goddess in Japanese mythology.

"This cosmic ray had about as much kinetic energy as a lead brick dropped from waist height, or about as much kinetic energy as a fast-pitched baseball," Matthews said. "That is a huge amount of energy in a sub-atomic particle. It is incredible!"

Intriguingly, it is not clear where the Amaterasu particle came from, or even exactly what it is. The team's findings show that its arrival direction corresponds to no obvious source galaxy, or any other known astronomical object thought to be potential sources of UHECRs.

"At the moment, we are still seeking sources or explanations for this event. It remains a mystery," Matthews said.

Nevertheless, researchers believe that extremely high energy cosmic rays should come from sources within the local universe—our cosmic neighborhood. At greater distances, they would be subject to collisions with the cosmic microwave background radiation—the faint remnant glow of the Big Bang—and lose their energy as a result.

Scientists are hoping that the latest discovery could shed new light on the origins of extremely energetic cosmic rays. The fact that the researchers could not identify any promising astronomical object in the direction that the Amaterasu particle came from suggests the possibility of unknown phenomena as a source, or that scientists have an incomplete understanding of the relevant high-energy particle physics.

"The Telescope Array has observed about 30 events with energy greater than [100 EeV]. The fact that we see a bunch of these events and they don't seem to point to sources is a puzzle," Matthews said. "There are obviously things we need to learn more about. Something strange is going on here."

Ke Fang, a researcher with the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin–Madison, who was not involved in the latest study, told Newsweek the Amaterasu particle shows how powerful "nature's accelerator" can be.

"Such a high energy is beyond the reach of man-made accelerators. Detecting and understanding these cosmic rays offers a unique probe of particle physics at extreme energies," Fang said.

David Saltzberg, a professor of physics and astronomy at the University of California, Los Angeles, who was also not involved in the study, told Newsweek the latest results were a "head-scratcher".

"Either it comes from close-by—in which case we ought to already be familiar with its source—or it comes from far away, in which case we know it could not stay so energetic. Paradoxes like these are what drive science forward," he said.

Update 11/27/23, 10:44 a.m. ET: This article was updated with additional comments from Ke Fang and David Saltzberg.