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

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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 detectors spanning 3,000 km2 of remote Argentina—complements the Telescope Array’s Northern Hemisphere vantage point. Together the two projects have found dozens of UHECRs over the years, yet the estimated energies of only a few—the original OMG particle and Amaterasu among them—have eclipsed 200 EeV. Statistics suggest such mighty messengers only arrive at a rate of less than one per century per square kilometer of the planet’s surface. Of those confirmed in astronomers’ catalogs, “you can count them on one hand,” says Noémie Globus of the University of California, Santa Cruz, who was a co-author of the new Science paper.

Studying a UHECR’s shower of secondary particles, scientists can reconstruct its crash-course trajectory to trace the likely path it took through space to pinpoint a possible astrophysical source. Such efforts have allowed researchers to search for shared sources via correlations between different UHECRs, with a few possible “hotspots” starting to emerge. Amaterasu complicates matters, however, because it appears to originate from the Local Void, a barren expanse of intergalactic space bordering the Milky Way. “The fact that it comes from this Local Void is really pretty puzzling,” says James Matthews of the University of Oxford, who wasn’t involved in the new finding.

Another layer of this puzzle is that no one knows exactly what sort of particle Amaterasu was—and different types of particles will have varying sensitivity to cosmic magnetic fields and background radiation that can bend their paths through space. If Amaterasu was a proton, as suggested by some experts, it would have been bent little and originated near the Local Void’s center. But if it were something heavier, such as the proton-and-neutron-packed nucleus of an iron atom, it would interact more strongly with magnetic fields, exhibiting a greater bend. In this scenario, Amaterasu’s origin could have been toward the Local Void’s edge, near a galaxy called NGC 6946.

John Matthews of the University of Utah, a co-author of the discovery paper, favors the proton explanation because of the composition and orientation of Amaterasu’s air shower. “Those things point to protons in this really high-energy range,” he says. That could suggest, in turn, that the source is one of the universe’s most energetic engines: supermassive black holes at the centers of “active” galaxies that feed on matter and fire out high-speed jets of protons and other subatomic particles. One nearby candidate is Centaurus A. At 13 million light-years away, Centaurus A is the closest active galaxy to Earth, and scientists have seen a potential clustering of some UHECRs there.

Others favor the heavier nuclei explanations. “If you asked me to bet on what it is, I would say it’s an iron nucleus,” says Glennys Farrar of New York University, who wasn’t involved in the new finding. The chief concern in that scenario would be how a bulky nucleus survives the brutal acceleration to relativistic speeds to become an extreme UHECR. “It’s bound together by a relatively weak amount of energy, compared to the process that’s accelerating it,” says David Kieda of the University of Utah, who co-discovered the original OMG particle. “It’s like trying to take a blob of Jell-O and speed it way up without destroying it.”

A so-called tidal disruption event in which a star is torn apart by a supermassive black hole could be one production route for an iron-nucleus UHECR, Farrar says. Such events are thought to be common among galaxies and could explain why UHECR sources are widely scattered across the sky, with only a few candidate hotspots. Perhaps Amaterasu’s source “just happens to be a galaxy where a star went fairly close to its supermassive black hole,” Farrar says. “I think that’s the most plausible explanation. You don’t need to have any tooth fairies.”

Scientists are busy upgrading both the Telescope Array and the Auger Observatory to hunt for answers. Plans are in place to expand the former to four times its current size in coming years, allowing more UHECR detections and better tracking to aid the hunt for any hotspots. Auger, meanwhile, is getting a crucial upgrade of radio antennas to augment its optical detectors. “[Radio] gives you a different signature for protons and iron,” Globus says, allowing researchers to discern between the two to winnow down probable astrophysical sources.

A proposed billion-dollar space telescope could vastly increase our understanding, too. Called the Probe of Extreme Multi-Messenger Astrophysics (POEMMA), it would train its eyes on Earth’s atmosphere from above—a lofty perch that would bring into view far more optical flashes from incoming UHECRs and perhaps increase the number of detections 10-fold. NASA has yet to green-light the project but is currently considering it for a potential launch opportunity in the 2030s. “They’ve got to convince NASA,” says Alan Watson, an emeritus professor at the University of Leeds in England, who set up the Auger Observatory and wasn’t involved in the new finding. “The competition for space experiments is so great.”

For now, the mystery remains; all that’s truly certain is that the hard rain of ultrahigh-energy cosmic rays will go on—and that we will continue to seek their enigmatic origins. Somewhere out there, at least one extraordinarily violent process is pushing the known boundaries of physics to send them our way. “These are just amazing events,” John Matthews says. “We’d like to know where they came from and how they got here.”

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