Astrophysicists using the Telescope Array experiment in Utah, the United States, have detected the second-highest extreme-energy cosmic ray.
In 1991, the High Resolution Fly’s Eye experiment detected the highest-energy cosmic ray ever observed.
Later dubbed the Oh-My-God particle, the cosmic ray’s energy shocked astrophysicists. Nothing in our Galaxy had the power to produce it, and the particle had more energy than was theoretically possible for cosmic rays traveling to Earth from other galaxies.
Led by the University of Utah and the University of Tokyo, the Telescope Array has since observed more than 30 ultra-high-energy cosmic rays, though none approaching the Oh-My-God-level energy. No observations have yet revealed their origin or how they are able to travel to the Earth.
On May 27, 2021, the Telescope Array experiment detected the second-highest extreme-energy cosmic ray.
At 2.4*1020 eV, the energy of this single subatomic particle is equivalent to dropping a brick on your toe from waist height.
The Telescope Array consists of 507 surface detector stations arranged in a square grid that covers 700 km2 outside of Delta, Utah.
The event triggered 23 detectors at the north-west region of the Telescope Array, splashing across 48 km2.
Its arrival direction appeared to be from the Local Void, an empty area of space bordering the Milky Way Galaxy.
“The particles are so high energy, they shouldn’t be affected by galactic and extra-galactic magnetic fields,” said Dr. John Matthews, Telescope Array co-spokesperson at the University of Utah.
“You should be able to point to where they come from in the sky.”
“But in the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there’s nothing high energy enough to have produced it.”
“That’s the mystery of this — what the heck is going on?”
The researchers named it the Amaterasu particle after the Sun goddess in Japanese mythology.
The Oh-My-God and the Amaterasu particles were detected using different observation techniques, confirming that while rare, these ultra-high energy events are real.
“These events seem like they’re coming from completely different places in the sky. It’s not like there’s one mysterious source,” said University of Utah’s Professor John Belz.
“It could be defects in the structure of spacetime, colliding cosmic strings. I mean, I’m just spit-balling crazy ideas that people are coming up with because there’s not a conventional explanation.”
Ultra-high-energy cosmic rays must exceed 5*1019 eV. This means that a single subatomic particle carries the same kinetic energy as a major league pitcher’s fast ball and has tens of millions of times more energy than any human-made particle accelerator can achieve.
Astrophysicists calculated this theoretical limit, known as the Greisen-Zatsepin-Kuzmin (GZK) cutoff, as the maximum energy a proton can hold traveling over long distances before the effect of interactions of the microwave background radiation take their energy.
Known source candidates, such as active galactic nuclei or black holes with accretion disks emitting particle jets, tend to be more than 160 million light-years away from Earth.
The new particle’s 2.41020 eV and the Oh-My-God particle’s 3.21020 eV easily surpass the cutoff.
The scientists also analyzed cosmic ray composition for clues of its origins: heavier particles, like iron nuclei, have more charge and are more susceptible to bending in a magnetic field than lighter particles made of protons from a hydrogen atom.
The new particle is likely a proton.
Particle physics dictates that a cosmic ray with energy beyond the GZK cutoff is too powerful for the microwave background to distort its path, but back tracing its trajectory points towards empty space.
“Maybe magnetic fields are stronger than we thought, but that disagrees with other observations that show they’re not strong enough to produce significant curvature at these ten-to-the-twentieth electron volt energies. It’s a real mystery,” Professor Belz said.
Sources:
Telescope Array Collaboration. 2023. An extremely energetic cosmic ray observed by a surface detector array. Science 382 (6673): 903-907; doi: 10.1126/science.abo5095
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