The Mysterious Origins of an Extraordinarily Energetic and Rare Cosmic Ray

A new partner to the "Oh-My-God" particle has been discovered.

The original "OMG" particle was observed by physicists in 1991 and displayed an impressive amount of energy at 320 quintillion electron volts, which is comparable to a baseball moving at a speed of around 100 kilometers per hour.

Scientists have now found a new particle with similar energy. In 2021, the Telescope Array experiment based near Delta, Utah, detected a particle with approximate energy of up to 240 exaelectron volts. There is a mystery surrounding this particle as researchers are unable to identify any cosmic source that could have created it.

The energy quantity recorded is massive, especially considering that it's contained in an incredibly tiny object, says astroparticle physicist John Matthews from the University of Utah in Salt Lake City, and also co-spokesperson of the Telescope Array collaboration.

Cosmic rays consist of protons and atomic nuclei that travel through space at varying energies. Highly energetic particles over 100 exaelectron volts are extremely rare; scientists estimate that such a particle strikes a square kilometer of the Earth's surface approximately once every century. Particles with over 200 exaelectron volts are even rarer, with only a few previously detected.

When a cosmic ray contacts Earth, it collides with an atom's nucleus in the atmosphere, creating a cascade of other detectable particles on the Earth's surface.

Scientists use large detector arrays to catch the rarest and highest-energy particles. The Telescope Array uses over 500 detectors made of plastic scintillator (a material that emits light when hit by a charged particle) covering an area of 700 square kilometers. Additional detectors measure the ultraviolet light from the particle shower in the sky, although these were not active when the new particle arrived, and 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 trace its origins.

Extremely high-energy cosmic rays are believed to originate from outside the Milky Way. However, their exact sources remain a mystery. Most scientists speculate that these particles are accelerated in violent cosmic environments, like the radiation jets from some supermassive black holes or from starburst galaxies that form stars at an extremely fast rate.

The highest-energy cosmic rays are believed to originate from the nearby cosmic neighborhood because these particles lose energy over time by interacting with the cosmic microwave background, which is essentially the afterglow of the Big Bang.

Tracing the particle’s origins is complex due to the magnetic fields present in the Milky Way and in its surroundings, which scatter the cosmic rays similar to how light gets scattered in fog. To track a particle's origin, these scattering effects must be accounted for. However, this backtracking process led to a cosmic void: regions of space with few galaxies and little violent activity.

This discovery leaves this particle particularly interesting, according to Vasiliki Pavlidou, an astrophysicist at the University of Crete in Heraklion, Greece. Despite the significant findings, the particle in question appears to have originated from "absolutely in the middle of nowhere", hinting at the possibility that scientists may need to extend their understanding of the galaxy's magnetic fields. Pavlidou, who was not part of this research, suggests that the rarity of such high-energy events means that each new detection is significant.