The Most Powerful Cosmic Ray Since the Oh-My-God Particle Puzzles Scientists

Astronomers have captured an extremely energetic cosmic particle whose cause and place of origin are a mystery.

Astronomers have captured a cosmic particle named “Amaterasu,” whose energy and origin pose a challenging explanation. In May 2021, this particle collided with Earth’s atmosphere, exhibiting an enormous energy of 244 exaelectronvolts, marking the most energetic event of its kind in the last three decades. Intriguingly, “Amaterasu” emerged from our local void, a rather desolate region of space where conventional explanations for the particle’s significant acceleration seem improbable, as reported in the journal Science.

Earth constantly experiences impacts from high-energy particles originating deep in space, known as cosmic radiation. While the radiation shielding of Earth blocks a sizable portion, some of these particles, when accelerated to extremely high speeds, enter the atmosphere. Upon collision with atmospheric gas particles, they initiate a cascade of secondary particles. Astronomers, equipped with specialized detectors, can trace and reconstruct the source, energy, and direction of cosmic radiation based on these particles.

Through Tscherenkow telescopes, it is now evident that the less energetic cosmic radiation primarily emanates from the sun, neighboring stars, supernovae, and even the center of the Milky Way.

Mystery Surrounding “Oh-My-God” and Co

high-energy phenomena in the cosmos
There are some extremely high-energy phenomena in the cosmos, but where the most energetic part of cosmic radiation comes from is still unknown. Image: Osaka Metropolitan University/ Kyoto University/ Ryuunosuke Takeshige.

However, this is different for high-energy cosmic radiation. Its particles travel through space with energies exceeding one exaelectronvolt, equivalent to a trillion electronvolts or millions times the collision energy in the Large Hadron Collider (LHC) at CERN. So far, only about two dozen such ultra-high-energy cosmic particles (UHECR) have been detected. Among them is the so-called “Oh-My-God” particle, which arrived in 1991 with an energy of 320 exaelectronvolts.

According to astronomers from Osaka Metropolitan University under the direction of Toshihiro Fuji, “the origin of such UHECR particles is typically believed to be in the most energetic phenomena of the universe, such as the relativistic outflows of black holes, gamma-ray bursts, or large-scale shockwaves from galaxy collisions.” While the high-energy particles seem to be less uniformly distributed than “normal” cosmic radiation, their origin remains unknown.

However, the sources of these UHECR particles cannot be very far away; because they interact with the cosmic background radiation, they lose energy during their journey. Therefore, even the most energetic UHECR can originate at a maximum distance of 160 to 320 million light-years.

New Particle with “Unprecedented Energy”

Fuji and his colleagues from the Telescope Array Collaboration report on a cosmic particle that presents multiple puzzles. The Telescope Array in Utah’s over 700 square kilometers of dispersed particle detectors managed to capture its signal on May 27, 2023. These detectors indicated that the originating particle must have possessed an enormous energy of 244 exa-electronvolts.

When I discovered this extremely high-energy cosmic ray, my initial thought was that there must be an error here because this energy level is unprecedented for the past 30 years,” says Fuji. The particle, named “Amaterasu,” could have been similarly energetic as the Oh-My-God particle of 1991, given the differences in energy measurement methods at that time, as explained by the team. Further analyses suggest that “Amaterasu” was likely a proton or a larger charged particle.

Where Does “Amaterasu” Come From?

Schematic of the Telescope Array in Utah: Three lateral fluorescence telescopes capture the radiation released by the secondary particle shower. The scintillators in between detect the particles.
Schematic of the Telescope Array in Utah: Three lateral fluorescence telescopes capture the radiation released by the secondary particle shower. The scintillators in between detect the particles.

“But where did this unusually high-energy particle come from? To find out, astronomers analyzed the angles at which the secondary particles hit the detectors. Because cosmic magnetic fields and other intervening influences hardly ever deflect particles with such high energies, it is possible to reconstruct at least the general direction of their trajectory. ‘One can then determine from where in the sky these particles originate,’ explains co-author John Matthews from the University of Utah.

The surprising result is that the Amaterasu particle seems to have come from the local Void, a relatively empty zone in space near our local galaxy group. ‘In this void, there are only a small number of galaxies, and none of them is a location where UHECR particles could have originated through acceleration,’ report the astronomers. Gamma-ray sources or other high-energy cosmic phenomena are also absent there. ‘This is truly a puzzle—what is happening here?‘ says Matthews. “

Is There “New Physics” Behind It?

Mysterious as well: The few Ultra-High-Energy Cosmic Ray (UHECR) events captured so far seem to originate from completely different directions. “There appears to be no clustering of these high-energy events,” explain the astronomers. Even the Oh-My-God particle came from a different part of the sky than Amaterasu does now. However, it has not yet been possible to identify a plausible source for any of these events.

Suggesting possibilities of unknown astronomical phenomena and novel physical origins beyond the Standard Model,” says Fuji. This could also include a yet-undiscovered type of particle that is not slowed down by cosmic background radiation and its magnetic influence. In that case, Amaterasu, Oh-My-God, and others could originate from a greater distance than the previously postulated maximum distance. However, the cause of these extremely high-energy particles in the exa-electronvolt range remains a puzzle for now.