Amaterasu – the most powerful cosmic ray seen in three decades – appears as if it comes from a blank spot in the sky. New telescopes may unravel the mystery of its origin.
The universe knows no shortage of ways to strike our planet
Between solar flares, quasars, gamma-ray bursts, and giant impacts, the universe suffers no shortage of ways to hit our planet. Among the strangest and most mysterious are ultra-high-energy cosmic rays (UHECRs), heavy and tiny particles from unknown places that collide with our planet at speeds approaching that of light. Typically, each ultra-high-energy cosmic ray arrives alone and without warning, like a speeding celestial bullet, striking our atmosphere and exploding into a shower of secondary particles that causes a flash of light unnoticed when they rain down on the surface. Although ground-based detectors have recorded a few ultra-high-energy cosmic rays by such “air showers” before, one that swept across the Utah sky in late spring 2021 was particularly intriguing. Named “Amaterasu” (the sun goddess in Japanese mythology) by its discoverers, this single cosmic ray seems to carry the energy equivalent of throwing a brick in atomic form, making it the most energetic particle seen on Earth in over 30 years. Strangely, it appears to have come from what amounts to “nowhere” – a vast expanse of cosmic void devoid of stars, galaxies, and most anything that could be an obvious astronomical source.
Amaterasu strikes Earth
Amaterasu hit the Earth in the early hours of May 27, 2021, leading to a shower of muons, gluons, and other secondary particles across 23 of over 500 sensors in the telescope array, a project stretching across 700 square kilometers of desert in Utah. By piecing together those particles, researchers inferred that the incoming cosmic ray must have an energy of about 244 exa-electron volts (EeV), equivalent to a well-thrown baseball and more than millions of times the energy of particles colliding at the Large Hadron Collider, the world’s strongest physics experiment. “I thought it must be a mistake,” says Toshihiro Fuji of Osaka Metropolitan University in Japan, who found the particle in the group’s data. But it was not. The results were published on November 23 in the journal Science.
The origin of Amaterasu and possible explanations
The complication is that no one knows exactly what type of particle Amaterasu was, and different types of particles would have varying sensitivities to cosmic magnetic fields and the background radiation that could bend their paths in space. If Amaterasu was a proton, as some experts have suggested, it would not bend much and would originate near the center of the local void. But if it were something heavier, like an iron nucleus made of protons and neutrons, it would interact more strongly with magnetic fields, showing greater curvature. In this scenario, Amaterasu’s origin could be toward the edge of the local void, near a galaxy called NGC 6946.
Upgrading current projects and the proposed space telescope
Scientists are working on upgrading both the telescope array and the Auger Observatory to search for answers. Plans are underway to expand the current array to four times its size in the coming years, allowing for better detection and tracking of ultra-high-energy cosmic rays to help search for any hotspots. Meanwhile, Auger is being upgraded with crucial radio antennas to enhance optical detection. “The radio gives you a different signature for protons vs. iron,” says Noemi Globus, allowing researchers to distinguish between the two to cut down on potential astronomical sources.
Can
The proposed space telescope, costing billions of dollars, will significantly enhance our understanding as well. It is called the “Extreme Universe Multi-Messenger Probe” (POEMMA), and it will train its eyes on Earth’s atmosphere from above – a high vantage point that brings in more optical flashes from incoming high-energy cosmic rays and could potentially increase detection rates by a factor of 10. NASA has not yet approved the project, but it is currently considering the possibility of launching it during a potential launch opportunity in the 2030s. “They have to convince NASA,” says Alan Watson, a retired professor at the University of Leeds in England, who founded the Auger Observatory and was not involved in the new discovery. “Competition for space experiments is very strong.”
Currently, the mystery remains; all that is really certain is that high-energy cosmic rain will continue – and that we will continue to search for its mysterious origins. Somewhere out there, at least, is an extraordinarily violent process pushing the boundaries of known physics to send it our way. “These are just amazing events,” says John Matthews. “We would like to know where they came from and how they got here.”
Copyright and Permissions: Jonathan O’Callahan is an award-winning freelance journalist covering astronomy and astrophysics, commercial spaceflight, and space exploration. Follow him on Twitter @Astro_Jonny
Leave a Reply