The Second Most Powerful Cosmic Ray in History Came from–Nowhere?

Stellar flares and supernovae, gamma-ray bursts and giant impacts—the universe has no shortage of ways to wallop our planet. Among the strangest and most mysterious are ultrahigh-energy cosmic rays (UHECRs), weighty but wee particles from parts unknown that occasionally slam into our planet at close to the speed of light. Each UHECR usually arrives alone and without warning, like a celestial speeding bullet, crashing into our atmosphere and exploding in a cascade of secondary particles that spark imperceptibly brief flashes of light as they rain down to the surface. Although Earth-based detectors have spotted a handful of extremely energetic UHECRs by such “air showers” before, one that ripped through the skies over Utah in the late spring of 2021 was especially intriguing. Dubbed “Amaterasu” (the goddess of the sun in Japanese mythology) by its discoverers, this single UHECR apparently packed the power of a thrown brick in its subatomic form, making it the most energetic particle seen on Earth in more than 30 years. Most curiously, it seems to have come from what amounts to nowhere—a vast region of cosmic emptiness bereft of stars, galaxies and most everything else that could be an obvious astrophysical source.

Amaterasu struck Earth in the early hours of May 27, 2021, sending an air shower of muons, gluons and other secondary particles into 23 of the more than 500 detectors of the Telescope Array, a project that sprawls across 700 square kilometers of desert in Utah. Piecing together those particles, researchers surmised that the incoming UHECR must have been some 244 exa-electron volts (EeV) in energy, equivalent to a well-pitched baseball and millions of times more energetic than particles crashed together in the Large Hadron Collider, the world’s most powerful physics experiment. “I thought it must be a mistake,” says Toshihiro Fujii of Osaka Metropolitan University in Japan, who found the particle in the array’s data. Yet it wasn’t. The findings were published on November 23 in the journal Science.

Only one other known UHECR exceeds Amaterasu in energy: the famed “Oh, my God particle,” or “OMG particle,” of 1991, which clocked in at 320 EeV. That record holder also struck Utah—not because of any cosmic grudge but simply because, then and now, Utah’s flat terrain and dark skies make it the Northern Hemisphere hub for UHECR-spying detectors. In the Southern Hemisphere the Pierre Auger Observatory—a network of 1,600…