A rare, extremely energetic cosmic ray has mysterious origins

The “Oh-My-God” particle has a new companion.

In 1991, physicists spotted a particle from space that crashed into Earth with so much energy that it warranted an “OMG!” With 320 quintillion electron volts, or exaelectron volts, it had the kinetic energy of a baseball zipping along at about 100 kilometers per hour.

Now, a new particle of comparable energy has been found, researchers report in the Nov. 24 Science. Detected in 2021 by the Telescope Array experiment near Delta, Utah, the particle had an energy of about 240 exaelectron volts. And mysteriously, scientists are unable to pinpoint any cosmic source for the particle.

“It’s a huge, huge amount of energy but in a tiny, tiny, tiny object,” says astroparticle physicist John Matthews of the University of Utah in Salt Lake City, co-spokesperson of the Telescope Array collaboration.

Cosmic rays consist of protons and atomic nuclei that zip through space at a wide range of energies. Particles with energies over 100 exaelectron volts are exceedingly rare: On average, scientists estimate, one such particle falls on a square kilometer of Earth’s surface each century. And particles over 200 exaelectron volts are even rarer — only a few such particles have previously been detected.

When a cosmic ray hits Earth, it collides with a nucleus of an atom in the atmosphere, creating a cascade of other particles that can be detected on Earth’s surface.

To catch the rarest, highest-energy particles, scientists build giant arrays of detectors. The Telescope Array monitors an area of 700 square kilometers using more than 500 detectors made of plastic scintillator, material that emits light when hit by a charged particle. Additional detectors measure ultraviolet light produced in the sky by the shower of particles (although those detectors weren’t operating during the newly reported particle’s arrival). Based on the times that individual scintillator detectors were hit by the cascade of particles, scientists can determine the direction of the incoming cosmic ray and use that information to trace it back to its origins.

Extremely high-energy cosmic rays come from outside the Milky Way, but their exact sources are unknown (SN: 9/21/17). Most scientists think they are accelerated in violent cosmic environments, such as the jets of radiation that blast out of the areas around certain supermassive black holes, or starburst galaxies that form stars at a frenetic pace. 

Whatever their origins, the particles must come from the relatively nearby cosmic neighborhood. That’s because the highest-energy cosmic rays lose energy as they travel, by interacting with the cosmic microwave background, the afterglow of the Big Bang (SN: 7/24/18).

Tracing back the particle’s location is complicated. “The issue is that when you detect a high-energy cosmic ray at Earth, the arrival direction that you get will not point to the source because it will be deflected by … any magnetic field that would be in the way,” says Telescope Array collaborator Noémie Globus, an astroparticle physicist at the University of California, Santa Cruz and the RIKEN research institute in Japan.  

The magnetic fields present in the Milky Way and its environs scatter the cosmic rays like fog scatters light. To trace the particle to its home, scientists must take that scattering into account. But that backtracking pinpointed a cosmic void, a region of space with few galaxies at all, much less ones with violent processes going on. 

That makes this particle particularly interesting, says astrophysicist Vasiliki Pavlidou of the University of Crete in Heraklion, Greece. “It’s actually pointing towards nothing at all, absolutely in the middle of nowhere.”

That might hint that scientist are missing something. For example, researchers may need to better understand the magnetic fields of the galaxy, says Pavlidou, who was not involved with the research. 

“Every time you have one of these very high-energy events, just because they are so rare, it’s a big deal.”