Picture yourself at an amusement park, hitting one of those boxing arcade machines. Think of the energy of that punch. Now imagine giving that energy to a single particle, which traveled across space from the local void and slammed into our planet’s atmosphere, showering the Telescope Array detector in secondary particles.
The particle was called Amaterasu, after the Japanese goddess of the Sun. It is an Ultra-high-energy cosmic ray (UHECRs), a subatomic particle with electric charge that was accelerated to incredible energies. Only one particle has been known to have a higher energy since we started to study these events, the Oh-My-God particle detected in 1991.
“When I first discovered this ultra-high-energy cosmic ray, I thought there must have been a mistake, as it showed an energy level unprecedented in the last three decades,” co-author Professor Toshihiro Fujii, from the Osaka Metropolitan University, said in a statement.
Usually, it is difficult to make comparisons between the energy of the particles in our particle accelerators and things we have direct experience with. Amaterasu and Oh-My-God make it very easy. They have 100 million times the energy of protons in the large hadron collider. Amaterasu had an energy of 244 Exaelectronvolts; Oh-my-god had an energy of 320 Exaelectronvolts. That’s equivalent to the aforementioned punch, dropping a brick onto your toe from waist height, or of a baseball in flight.
The incredible energy means that it roughly came in a straight line to Earth – nothing to deflect it – and this is a problem, because at the other end, there is literally nothing.
“The particles are so high energy, they shouldn’t be affected by galactic and extra-galactic magnetic fields. You should be able to point to where they come from in the sky,” co-author John Matthews, Telescope Array co-spokesperson at the University of Utah, said in a statement.
“But in the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there’s nothing high energy enough to have produced it. That’s the mystery of this – what the heck is going on?”
Amaterasu comes from the Local Void, a region of space that might extend for about 200 million light-years and whose center is at least 75 million light-years from Earth. As the name suggests, it is a mostly empty region, with far fewer galaxies than the rest of the local universe.
But something must have produced the Oh-My-God and the Amaterasu particles. The two were detected using different observation techniques. Amaterasu thanks to the Telescope Array located in the Utah desert. These particles are rare but they are definitely real.
“These events seem like they're coming from completely different places in the sky. It’s not like there's one mysterious source,” added John Belz, professor at the University of Utah and co-author of the study. “It could be defects in the structure of spacetime, colliding cosmic strings. I mean, I’m just spit-balling crazy ideas that people are coming up with because there's not a conventional explanation.”
We have seen some astronomers start taking bets on the possible sources (if someone has the odds on magnetars, let us know). The Telescope Array is being expanded and researchers are hopeful that they’ll discover more of these extreme particles and maybe their origin too.
A paper describing the discovery is published in the journal Science.