The Earth is constantly hit by high-energy particles from deep space - cosmic rays. But on May 27, 2021, a network of telescopes in the USA captured the signal of a cosmic particle that is puzzling astronomers. Because this particle, dubbed “Amaterasu,” collided with the Earth’s atmosphere with the enormous energy of 244 exaelectron volts – ten million times more than even the most powerful particle accelerator on Earth can generate. This means that this particle belongs to only a handful of events at this energy level. What releases these cosmic particles is still completely unclear. The origin of the Amaterasu particle is also puzzling: it appears to have come from an almost empty zone of our universe.
Cosmic rays are omnipresent in space. However, on Earth, a large proportion of these high-energy charged particles are intercepted by the magnetic field and atmosphere. Still, some penetrate so far that they can collide with gas atoms in the atmosphere and create a whole shower of secondary particles. Their quantity, energy and direction make it possible to at least begin to reconstruct what kind of particle hit us, with what energy and from what direction. Particles with rather lower energies appear to emanate from the sun, neighboring stars and also from the center of the Milky Way. Cosmic radiation is also released in the form of accelerated particles during supernovae or black holes. However, the origin of the most energetic portion of cosmic radiation is largely unclear. These particles race through space with energies of more than one exaelectron volt - this corresponds to a trillion electron volts or a million times the collision energy of protons in the Large Hadron Collider (LHC) particle accelerator at the CERN research center.
Energies in the exaelectronvolt range
So far, astronomers have only detected around two dozen cosmic particles that belong to this ultra-high-energy cosmic ray (UHECR). “At energies of more than 100 exaelectronvolts, the influx is less than one particle per century per square kilometer,” explain astronomers from the Telescope Array Collaboration. One of the few events of this kind is the so-called Oh My God particle, which was detected in 1991 and had an energy of 320 exaelectron volts. “According to common assumptions, the origin of such UHECR particles lies in the most energetic phenomena in the universe, such as the relativistic outflows of black holes, gamma ray bursts or large-scale shock waves from galaxy collisions,” explain the astronomers. However, the distance over which the particles can maintain such high energy is limited: they interact with the microwave background of the cosmos and are thereby slowed down. According to current theory, the most energetic UHECRs can therefore come from a maximum distance of 160 to 320 million light years.
But now astronomers led by Toshihiro Fuji from Osaka Metropolitan University are reporting another record-breaking event. On May 27, 2023, the particle detectors of the Telescope Array in Utah, spread over an area of 700 square kilometers, captured the secondary particle shower of a cosmic particle that had the enormous energy of 244 exaelectron volts. “When I discovered this extremely energetic cosmic ray, I initially thought there must be something wrong because it showed an energy level that was unprecedented in the last 30 years,” says Fuji. Only the Oh My God particle was a little more energetic. Further analysis suggested that the current particle, named “Amaterasu” after the Japanese sun goddess, was probably a proton or larger charged particle.
Where do these particles come from?
To find out where the Amaterasu particle came from, astronomers evaluated the angles at which the secondary particles hit the detectors. Because particles with such high energies are hardly deflected by cosmic magnetic fields and other influences in their path, at least the rough direction of flight can be determined from this
reconstruct. “You can then find out where in the sky these particles come from,” explains co-author John Matthews from the University of Utah. However, when astronomers reconstructed the trajectory of the Amaterasu particle, something surprising emerged: the particle appeared to come from the local void - a relatively empty zone of space near our local group of galaxies. “There are only a small number of galaxies in this void, and none of these are places where UHECR particles could have formed through acceleration,” the astronomers write. In the void there are neither gamma ray sources nor other cosmic phenomena that generate enough energy. Although there is an active galaxy in the direction of the trajectory, it is almost two billion light-years away and therefore beyond the maximum distance for such ultra-energy-rich particles.
“It’s really a mystery – what’s going on here?” says Matthews. In addition, the few UHECR events captured so far seem to come from very different directions. The Oh My God particle also came from another part of the sky. “There is apparently no concentration of these high-energy events,” explain the astronomers. What causes these extremely high-energy particles in the exaelectronvolt range remains a mystery for the time being.
Source: Telescope Array Collaboration, Science, doi: 10.1126/science.abo5095