Astronomers shocked by mysterious, extremely high-energy cosmic rays – ‘What the hell is going on?’

Second only to the Oh-My-God particle, the newly named Amaterasu particle deepens the mystery of the origin, propagation and particle physics of rare, ultra-high-energy cosmic rays.

In 1991, the University of Utah’s Fly’s Eye experiment revealed the highest-energy cosmic rays ever observed. Later dubbed the Oh My God particle, the energy of cosmic rays shocked astrophysicists. Nothing in our galaxy had the capacity to produce that, and the particle had more energy than was theoretically possible for cosmic rays traveling to Earth from other galaxies. Simply put, the particle shouldn’t be there.

Astronomical puzzles

The telescope array has since detected more than 30 high-energy cosmic rays, although none approached the energy level. No observations have yet revealed their origin or how they were able to travel to Earth.

Artist’s illustration of highly energetic cosmic rays observed by the Array Telescope’s surface detector array, called Amaterasu particles. Source: Osaka Metropolitan University/L-Insight, Kyoto University/Ryuunosuke Takeshige

On May 27, 2021, the Telescope Array experiment detected the second highest energy cosmic ray. Size 2.4 x 10²⁰Volt, the energy of this single subatomic particle is equivalent to dropping a stone on your toe from waist height. The telescope array, led by the University of Utah (U) and the University of Tokyo, consists of 507 surface detection stations arranged in a square grid covering 700 km² (~270 miles²) outside of Delta, Utah in the state’s western desert. This event triggered 23 detectors in the northwestern region of the telescope array, spread across a distance of 48 km.² (18.5 miles²). Its direction of arrival appears to have been from the local void, an empty region of space adjacent to milky way galaxy.

“The particles are very high energy, and should not be affected by galactic and extragalactic magnetic fields. They should be able to point to where in the sky they came from,” said John Matthews, a spokesperson for the Telescope Array at the University of California and co-author of the study. But in the case of the Oh My God particle and this new particle, you can trace it back to its source and there’s nothing high enough energy to produce it. That’s the secret of this – what the hell is going on?

Amaterasu particle

In their note published on November 24, 2023 in the magazine Sciences, an international collaboration of researchers described extremely high-energy cosmic rays, evaluated their properties, and concluded that rare phenomena may follow particle physics unknown to science. The researchers named it the Amaterasu particle, after the sun goddess in Japanese mythology. Oh-My-God and Amaterasu particles were discovered using different observational techniques, confirming that these extremely high-energy events, although rare, are real.

Artistic illustration of ultra-energy cosmic ray astronomy to illustrate highly energetic phenomena in contrast to weaker cosmic rays affected by electromagnetic fields. Source: Osaka Metropolitan University/Kyoto University/Ryuunosuke Takeshige

“These events seem to come from completely different places in the sky. It’s not as if there’s one mysterious source,” said John Bales, a professor at the U and co-author of the study. “It could be imperfections in the structure of space-time, collisions of cosmic strings. I mean, I’m just talking about the crazy ideas that people come up with because there’s no conventional explanation.

Natural particle accelerators

Cosmic rays are echoes of violent celestial events that stripped matter down to its subatomic structure and flung it across the universe at nearly the speed of light. Cosmic rays are basically charged particles with a wide range of energies consisting of positive protons, negative electrons, or entire atomic nuclei that travel through space and fall on the Earth almost continuously.

Cosmic rays hit the Earth’s upper atmosphere and explode the nuclei of oxygen and nitrogen gas, generating many secondary particles. These particles travel a short distance into the atmosphere and repeat the process, creating a shower of billions of secondary particles that scatter over the surface. The area of ​​this secondary shower is enormous and requires the detectors to cover an area as large as a telescope array. Surface detectors use a combination of devices that provide researchers with information about each cosmic ray; The timing of the signal shows its path and the amount of charged particles hitting each detector reveals the energy of the elementary particle.

Because the particles have a charge, their flight path resembles a ball in a pinball machine as it zigzags against electromagnetic fields through the cosmic microwave background. It is almost impossible to trace the path of most cosmic rays, which lie on the low to medium end of the energy spectrum. Even high-energy cosmic rays are distorted by the microwave background. Particles containing Oh-My-God and Amaterasu’s energy blast through intergalactic space in a relatively unbent shape. Only the most powerful celestial events can produce them.

“Things that people think are active, like supernovas, are nowhere near energetic enough to do that. We need huge amounts of energy, and very high magnetic fields to confine the particle as it accelerates,” Matthews said.

The secret of ultra-energy cosmic rays

High energy cosmic rays must exceed 5 x 10¹⁹ Volt. This means that a single subatomic particle carries the same kinetic energy as a major fielder’s fastball, and has tens of millions of times more energy than any man-made particle accelerator could achieve. Astrophysicists have calculated this theoretical limit, known as the Gryssen-Zatsepian-Kuzmin (GZK) cutoff, as the maximum amount of energy a proton can carry while traveling long distances before microwave background radiation interactions take away its energy. Known candidate sources, such as active galactic nuclei or black holes with accretion disks emitting particle jets, tend to be more than 160 million light-years from Earth. The new particle is 2.4×10²⁰ A volt and a particle, oh my, 3.2 x 10²⁰ Volt easily exceeds the cutoff.

Researchers are also analyzing the composition of cosmic rays for clues about their origins. Heavier particles, such as iron nuclei, are heavier, have a greater charge, and are more likely to bend in a magnetic field than lighter particles made of hydrogen protons. corn. The new particle is likely to be a proton. Particle physics dictates that a cosmic ray with energy exceeding the GZK cutoff is so strong that the microwave background cannot distort its path, but it traces its path points into empty space.

“Magnetic fields may be stronger than we thought, but this is inconsistent with other observations that show they are not strong enough to produce significant curvature at energies of 10 to 20 MeV,” Bales said. “It’s a real mystery.”

Expand the search and the telescope array

the Telescope array It is uniquely positioned to detect very high energy cosmic rays. It is located at an altitude of about 1,200 meters (4,000 ft), the ideal elevation point that allows secondary particles to develop maximum, but before they begin to decay. Its location in Utah’s western desert provides ideal weather conditions in two ways: dry air is critical because moisture will absorb the ultraviolet light needed for detection; Dark skies are necessary, because light pollution will create a lot of noise and block cosmic rays.

Astrophysicists are still puzzled by this mysterious phenomenon. The telescope array is in the middle of an expansion process that they hope will help solve this issue. Once completed, 500 new scintillating detectors will expand the telescope’s array and sample showers of particles generated by cosmic rays across 2,900 kilometers.² (1100 miles² ), an area about the size of the state of Rhode Island. Hopefully the larger footprint will capture more events that will shed light on what’s going on.

For more on this discovery:

Reference: “An extremely energetic cosmic ray detected by a surface detector array” November 23, 2023, Sciences.
doi: 10.1126/science.abo5095