The University of Utah Fly’s Eye experiment discovered the highest-energy cosmic ray ever observed in 1991. The energy of the cosmic ray, later dubbed the Oh-My-God particle, astrophysicists. Nothing in our galaxy had the power to create it, and the particle had more energy than cosmic rays traveling to Earth from other galaxies. To put it simply, the particle should not exist.
Since then, the Telescope Array has detected more than 30 ultra-high-energy cosmic rays, though none have approached the Oh-My-God level of energy. No observations have revealed their origins or how they can travel to Earth.
The Telescope Array experiment discovered the second-highest extreme-energy cosmic ray on May 27, 2021. The energy of this single subatomic particle is 2.4 x 1020eV, which is equivalent to dropping a brick on your toe from waist height. The Telescope Array, led by the University of Utah (the U) and the University of Tokyo, consists of 507 surface detector stations arranged in a square grid covering 700 km2 (~270 miles2) outside of Delta, Utah in the state’s West Desert. The event triggered 23 detectors in the Telescope Array’s northwestern region, covering 48 km2 (18.5 mi2). It appeared to have arrived from the Local Void, an empty region of space bordering the Milky Way galaxy.
Things that people think of as energetic, like supernova, are nowhere near energetic enough for this. You need huge amounts of energy, really high magnetic fields to confine the particle while it gets accelerated.
John Matthews
“The particles are so high energy, they shouldn’t be affected by galactic and extra-galactic magnetic fields. You should be able to point to where they come from in the sky,” said John Matthews, Telescope Array co-spokesperson at the U and co-author of the study. “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?”
An international collaboration of researchers describes the ultra-high-energy cosmic ray, evaluates its characteristics, and concludes that the rare phenomenon may follow particle physics unknown to science in their observation published on Nov. 24, 2023, in the journal Science. The Amaterasu particle was named after the sun goddess in Japanese mythology by the researchers. Using various observation techniques, the Oh-My-God and Amaterasu particles were detected, 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 John Belz, professor at the U and co-author of the study. “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.”
Natural particle accelerators
Cosmic rays are echoes of violent celestial events that have stripped matter to its subatomic structures and hurled it through universe at nearly the speed of light. Essentially cosmic rays are charged particles with a wide range of energies consisting of positive protons, negative electrons, or entire atomic nuclei that travel through space and rain down onto Earth nearly constantly.
Cosmic rays collide with Earth’s upper atmosphere, rupturing the nuclei of oxygen and nitrogen gas and generating a slew of secondary particles. These particles travel a short distance in the atmosphere before repeating the process, resulting in a shower of billions of secondary particles that scatter to the surface. This secondary shower has a massive footprint, requiring detectors to cover an area the size of the Telescope Array. The surface detectors use a suite of instruments that provide researchers with information about each cosmic ray; the timing of the signal shows its trajectory, and the amount of charged particles hitting each detector reveals the energy of the primary particle.
Because particles have a charge, their flight path resembles a ball in a pinball machine as they zigzag against the electromagnetic fields through the cosmic microwave background. It’s nearly impossible to trace the trajectory of most cosmic rays, which lie on the low- to middle-end of the energy spectrum. Even high-energy cosmic rays are distorted by the microwave background. Particles with Oh-My-God and Amaterasuenergy blast through intergalactic space relatively unbent. Only the most powerful of celestial events can produce them.
“Things that people think of as energetic, like supernova, are nowhere near energetic enough for this. You need huge amounts of energy, really high magnetic fields to confine the particle while it gets accelerated,” said Matthews.
Ultra-high-energy cosmic rays must have an energy greater than 5 x 1019 eV. This means that a single subatomic particle has the same kinetic energy as a major league pitcher’s fast ball and tens of millions of times more energy than any human-made particle accelerator. Astrophysicists calculated the Greisen-Zatsepin-Kuzmin (GZK) cutoff as the maximum energy a proton can hold while traveling over long distances before the effect of interactions with microwave background radiation takes their energy. Known source candidates, such as active galactic nuclei or black holes with accretion disks emitting particle jets, are typically more than 160 million light years away from Earth. The new particle’s 2.4 x 1020 eV and the Oh-My-God particle’s 3.2 x 1020 eV easily surpass the cutoff.
Researchers also analyze cosmic ray composition for clues of its origins. A heavier particle, like iron nuclei, are heavier, have more charge and are more susceptible to bending in a magnetic field than a lighter particle 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,” said Belz. “It’s a real mystery.”
Expanding the footprint
The Telescope Array is in an excellent position to detect ultra-high-energy cosmic rays. It is located at an elevation of about 1,200 m (4,000 ft), which is the sweet spot for secondary particle development before they begin to decay. Its location in Utah’s West Desert provides ideal atmospheric conditions in two ways: the dry air is important because humidity absorbs the ultraviolet light required for detection, and the region’s dark skies are important because light pollution creates too much noise and obscures the cosmic rays.
The mysterious phenomenon continues to perplex astronomers. The Telescope Array is in the midst of an expansion that they hope will aid in the investigation. Once completed, 500 new scintillator detectors will expand the Telescope Array to sample cosmic ray-induced particle showers over an area nearly the size of Rhode Island, measuring 2,900 km2 (1,100 mi2). The larger footprint should capture more events, shedding light on what’s going on.
