The Second Most Energetic Cosmic Ray Ever Found

“Oh My God,” someone must have said in 1991 when researchers detected the most energetic cosmic ray ever to strike Earth. Those three words were adopted as the name for the phenomenon: the Oh-My-God particle. Where did it come from?

Some unknown, extraordinarily powerful event out there in the cosmos sent this single particle all the way to Earth as a signal of its occurrence.

Nobody knows where it came from nor what type of particle Oh-My-God was. Cosmic rays are typically protons; whatever this particle was, it was extraordinarily energetic. It had 40 million times more energy than anything scientists have propelled in any particle accelerator: about 320 million TeV (tera electron volts.)

The University of Utah Fly’s Eye detected the Oh-My-God Particle (OMGP). The same facility detected many more cosmic rays, but none approaching the OMGP’s energy. Now, researchers have detected another ultra-energetic cosmic ray. It was detected with the Telescope Array Project, another ultra-high energy cosmic ray detector, also in Utah, and the Fly’s Eye’s successor.

The detection is presented in new research published in the journal Science. The research article is “An Extremely Energetic Cosmic Ray Observed by a Surface Detector Array.” John Matthews is the Telescope Array co-spokesperson at the University of Utah and co-author of the study.

“I’m just spit-balling crazy ideas that people are coming up with because there’s not a conventional explanation.”

John Belz, study co-author and professor at the University of Utah

Here’s what happens with cosmic rays and how scientists observe them.

Somewhere in the cosmos, a high-energy event such as a supernova releases particles with extremely high energies. The particles are usually protons, and they move at speeds approaching the speed of light. When they strike Earth’s atmosphere, they produce showers of secondary particles. The particles can be detected directly by satellites or high-altitude balloons or indirectly by ground stations that measure the secondary particles produced in the shower.

Artist’s impression of cosmic rays striking Earth and the resulting shower of particles. Image Credit: Simon Swordy/University of Chicago, NASA

The cosmic rays and their showers are kind of like what happens in a particle accelerator. But the energy of cosmic rays is off the charts. While the Large Hadron Collider can produce protons with 14 TeV, the OMGP is 40 million times more energetic.

This most recent detection is the second highest-energy cosmic ray ever detected and was detected in May 2021. This single proton had as much energy as dropping a brick on your foot from waist height: 240 exa–electron volts. It doesn’t rival the OMGP, but it’s more than a million times more energetic than anything our particle accelerators can generate.

This simple schematic shows a proton colliding with a particle in Earth’s atmosphere, producing a shower of secondary particles. Image Credit: By SyntaxError55 at the English Wikipedia, CC BY-SA 3.0, https://ift.tt/RDNASvX

“Its arrival direction points back to a void in the large-scale
structure of the Universe.”

From “An extremely energetic cosmic ray observed by a
surface detector array”

The confounding thing about these high-energy particles is their source. While supernovae may be a source of some of them, there’s uncertainty if any supernovae are actually powerful enough to produce the most energetic ones. The most energetic cosmic particles are travelling so quickly and with so much energy that nothing should be able to affect their trajectory, not even magnetic fields. So scientists should be able to look backward along their path straight to their source. But this one looks like it came from a void. “Its arrival direction points back to a void in the large-scale structure of the Universe,” the authors write in their paper.

“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, 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?”

This figure from the study shows the arrival directions (empty circles) of all >100 EeV cosmic rays observed by the Telescope Array during 13.5 years of operation. They’re spread around the sky, and no clustering around the highest energy event (thick circle) is evident. “Although we may have expected events with energies above 100 EeV to be clustered, the observed arrival directions above 100 EeV have an isotropic distribution,” the authors explain. Image Credit: Telescope Array.org

Whatever they are, the particle showers they create cover a wide area. It takes an array of detectors to measure one of these showers. The Telescope Array Project is made of three telescope detectors and associated equipment. The telescopes are 35 km apart from one another.

The new particle, named the Amaterasu particle after the sun goddess in Japanese mythology, is unexplained so far. But it was detected with a different method than the method used to detect the Oh-My-God Particle. That helps confirm that these particles are indeed 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 University of Utah 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.”

For something that constantly bombards Earth, there are huge gaps in our knowledge of cosmic rays. Something violent happens out there in the Universe, stripping matter of its subatomic structure and sending the particles hurling outward through space. The particles can be positive protons, negative electrons, or complete atomic nuclei.

A supernova is one of the most cataclysmic, explosively energetic natural events we know of. But even they aren’t powerful enough to produce Amaterasu.

“Things that people think of as energetic, like supernovae, 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. Supernovae can produce cosmic rays with lower energies, but not the monstrously energetic ones like Amaterasu and the Oh-My-God particle.

Why doesn’t Amaterasu point back to its source? Researchers are wracking their brains trying to find an answer. Could magnetic fields be stronger than thought, and could they be altering the trajectories of these ultra-high energy cosmic rays (UHERCs)?

“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.”

“The arrival direction of this event does not align with any known astronomical objects thought to be a potential source of UHECRs, even after taking into account deflection by the GMF (Galactic Magnetic Field) under various assumptions,” the authors write in their paper’s conclusion.

Like so many things in astronomy and astrophysics, better detectors might lead to an answer. The Telescope Array is being expanded and will cover an area nearly the size of Rhode Island. With a much-coveted larger sample, more detections might bring us closer to an answer.

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