Physicists Are Racing To Pinpoint the Origin of An Extremely Energetic Cosmic Ray

An incredibly fast-moving particle from space recently arrived on Earth, and physicists are rushing to figure out what it is and where it came from.

Called the Amaterasu particle, after the Shinto goddess whose name means “The Great Divinity That Illuminates Heaven,” is one of several mystery particles, called ultra-high energy cosmic rays that physicists have detected in the past few decades.

Osaka Metropolitan University astrophysicist Toshihiro Fujii and his colleagues at the Telescope Array Experiment published their findings in the journal Science.

A High-Energy Arrival

In May 2021, instruments spread across the Utah desert detected a cascade of subatomic particles. The particle shower meant that somewhere above the 270-square-mile swath of the Telescope Array Experiment, something tiny but with tremendous energy had just hit Earth’s atmosphere. When it slammed into Earth’s atmosphere, it broke apart into a shower of what physicists call secondary particles: things like photons, electrons, positrons, and in this case, a lot of muons.

When Fujii saw the data for the first time, he thought there must have been a mistake.

The incoming object that Fujii and his colleagues’ instruments detected was a cosmic ray, a fast-moving, high-energy subatomic particle — either the nucleus of an atom or something even smaller. Physicists aren’t completely sure where cosmic rays come from, but most agree that they’re accelerated to nearly the speed of light by powerful magnetic fields around supernovae or black holes. Cosmic rays patter against the upper levels of Earth’s atmosphere fairly often, and scientists study them by measuring the shower of secondary particles that rains down after a cosmic ray breaks apart.

Most of those normal cosmic rays carry around one exa-electronvolt (EeV) of energy, which is about a million times more than the most powerful human-made particle accelerators, like the Large Hadron Collider, can produce. But the one Fujii and his colleagues detected in May 2021 carried 244 EeV of energy — which is, to put it mildly, an absolutely off-the-charts outlier of a number. Something out there in space had accelerated the living heck out of this tiny subatomic particle and flung it in our direction.

Fujii and his colleagues named their cosmic ray Amaterasu after a Shinto goddess associated with the creation of Japan, but the naming was posthumous, of course — the original particle shattered into all those secondary particles in the upper atmosphere. But for that brief moment, it was the second most energetic cosmic cosmic ray that physicists have ever detected.

A Mysterious Origin Story

Particles don’t usually zip around space with that much energy — something has to accelerate them. Physicists have traced at least some cosmic rays back to a supernova remnant, for instance. But a mere supernova can’t accelerate a particle to a high enough energy to match Amaterasu or other ultra-high-energy cosmic rays. That takes something like a growing supermassive black hole at the heart of a galaxy or the turbulent “accretion shock” near the borders of a galaxy cluster.

“However, so far we have not been able to collect enough data to convincingly identify sources,” says University of Utah astrophysicist John Matthews, a co-author of the recent paper, tells Inverse.

Fujii and his colleagues are still trying to figure out where Amaterasu came from, and to do that, they’ll need to figure out its composition.

Most of the cosmic rays that reach Earth are single protons, but some cosmic rays are the nuclei of larger, heavier atoms that are stripped of their electrons. These heavier nuclei have stronger electrical charges (because they’ve got more positively charged protons), so our galaxy’s magnetic field can pull them farther off-course. So if you’re a physicist trying to trace a cosmic ray back to its source, you need a decent map of our galaxy’s magnetic field, and you need to know what kind of cosmic ray we’re talking about.

A map published by Fujii and his colleagues illustrates how big a difference both of those things can make. The team compared two different models of the galactic magnetic field, along with several possible identities for Amaterasu. If the cosmic ray was a single proton, then it seems to have come from an area called the Local Void, which is exactly what it sounds like. But if it was the stripped-bare nucleus of a carbon, silicon, or iron atom, then the models put its origin point near a couple of different possible sources – depending on which magnetic field model you prefer (and even physicists collaborating on the same paper don’t agree on that sort of thing very often).

“For me, since we don't see a source directly behind it, it's likely to be heavier than a proton,” University of California, Santa Cruz astrophysicist Noemie Globus, also a co-author of the recent paper, tells Inverse. “But it could be, you know, intermediate mass like nitrogen or even iron because we still don't know the composition of cosmic rays at the highest energies.”

Usually, physicists can reconstruct a cosmic ray’s original makeup based on the particles in its secondary shower. For more common, lower-energy cosmic rays, physicists have computer models that can simulate what that rain of tiny subatomic particles should look like, depending on the energy and makeup of the original cosmic ray. All they have to do is compare their simulations to the real data and figure out which model offers the closest fit.

But only a handful of high-energy cosmic rays like Amaterasu have ever been detected, so physicists don’t have that much information to put into their models.

“If we knew very well the physics at these high energies, we would do better,” says Globus.

Turns out high-energy particle physics is challenging even for high-energy particle physicists.

Putting the Cosmic Pieces Together

The most energetic cosmic ray ever detected shattered to bits in Earth’s atmosphere in 1991 with an energy of 320 EeV, which is only a few hundred million Large Hadron Colliders’ worth of energy. Physicists at the time named it the Oh-My-God Particle, and more than 30 years later, we still don’t know its source or composition.

“People have looked into the Local Void, where the Amaterasu particle came from and also the direction of the ‘Oh-My-God’ particle, and so far, they have not been able to find a violent object that could be the source of these particles,” says Matthews. “At a distance less than 100 megaparsecs, these should be observable.” Cosmic rays have trouble traveling any farther than 100 megaparsecs because they eventually tend to crash into photons from the Cosmic Microwave Background.

That suggests that it’s time to rethink either the composition of these speedy, energetic particles or the map of our galaxy’s magnetic field. A team of physicists recently published a preprint paper that updates one of the models Fujii and his colleagues used, and members of the Telescope Array Experiment are working on ways to combine machine learning with new types of observations to unravel the mystery of ultra-high-energy cosmic rays.

Meanwhile, as is often the case, the best answer is just more data.

“We need to have more and more particles, to be able to start to see if some directions are more likely to be associated with sources,” says Globus.

Editors Note: This story has been updated to reflect the fact that Noemie Globus is an astrophysicist at the University of California, Santa Cruz, not the University of Utah, as previously stated. We are glad to correct the error.