Cosmic ray particles come in very different sizes. Protons are light, but iron nuclei are heavy. Scientists have long assumed that heavier particles would behave differently from lighter ones as their energies climb: different masses, different ceilings.
A satellite has now spent nine years tracking them from orbit, and its newest data challenges that assumption.
Every type of nucleus – heavy or light – hits a drop-off at the exact same point. That pattern may indicate where they come from.
A century-old puzzle
Cosmic rays are high-energy particles that travel through space at nearly the speed of light. The fastest ones carry more energy than anything we can build on Earth, including particles produced in the Large Hadron Collider.
Most of the particles are protons or heavier atomic nuclei stripped down to bare cores.
“Cosmic rays are primarily composed of protons, but also of helium, carbon, oxygen, and iron nuclei,” said Andrii Tykhonov, an associate professor at the University of Geneva (UNIGE).
They’ve been studied since 1912, when physicist Victor Hess found that cosmic rays grew stronger at altitude. He rode a balloon to 17,000 feet (5,180 meters) to prove it.
Despite decades of research on cosmic rays, however, important questions have lingered. How do these ultra-powerful particles get cranked up to such extreme energies, and where, exactly, do they come from?
Tykhonov is one of the researchers chasing those answers.
Following cosmic particles
The instrument they’re using is called the DAMPE space telescope, short for Dark Matter Particle Explorer.
Since it was launched in December 2015, it’s been quietly orbiting Earth, sweeping up particles as they slam into its detector stack.
The project is Chinese-led, with critical hardware built in part by Tykhonov’s group in Geneva. Their instrument picks out a particle’s direction of travel and electric charge.
Without that, telling a fast proton from a fast iron nucleus would be guesswork. The new study draws on nine years of clean data.
Where the spectrum bends
Plot the number of cosmic rays against their energy and you get a spectrum. Lower-energy particles are common, higher-energy ones rare, and the curve drops steadily as particle energy climbs. That much has been known for decades.
But at a certain energy the decline kicks into a steeper drop. The line bends down faster than before and there are fewer particles than expected.
Physicists call this spectral softening. DAMPE has seen it cleanly across all cosmic ray nuclei from light protons up to heavy iron.
For the heavier elements, this is the first direct measurement.
Universal cosmic limit
What surprised everyone is that the bend happens at the same place for every nucleus. Not the same energy, but the same rigidity – a measure of how easily magnetic fields can bend a particle’s path.
Convert from energy to rigidity, and protons, helium, carbon, oxygen, and iron all start falling off faster at roughly 15 teravolts. An earlier analysis had hinted at this for protons alone.
Different masses, different charges, different properties – and yet the curve breaks at one common point. Now the pattern shows up across the board.
Charge or mass
There are two main camps explaining where cosmic rays come from. One says the energy ceiling depends on a particle’s electric charge – how strongly magnetic fields grip it during acceleration.
The other says the ceiling depends on mass. Different nuclei interact differently on the long journey through interstellar gas. Two frameworks, two very different predictions for where the curve breaks.
The new data backs the charge camp and rules out the mass-based version with better than 99.999% confidence. A century of competing theories, narrowed in nine years of orbit.
AI traces the tracks
Untangling a single cosmic ray’s path through silicon and tungsten is not easy. What arrives is a tangled spray of smaller particles, created on impact, and the original direction has to be reconstructed from the pieces.
Geneva’s team built a deep-learning system to do exactly that. It was trained on simulated collisions first, then deployed on real data.
They also chased rare iron nuclei, which arrive far less often than protons. Filtering iron nuclei out from billions of lighter cousins demands careful calibration and patient cross-checks. The reward was the first clear softening signal ever measured for iron at these energies.
Tracing the cosmic origin
So what’s making the curve bend at 15 teravolts? The simplest reading is that whatever cosmic accelerator launches these particles can only push them up to a certain rigidity.
Supernova explosions have been the prime suspect for almost a century. Their blast waves can whip ionized atoms to extreme energies, and there’s solid evidence of that from individual remnants.
The DAMPE team suggests the break could trace back to a single, relatively close source. This could be an old supernova whose lingering halo still washes over the solar system today.
What changes now
Researchers can now name a specific number – roughly 15 teravolts – as where cosmic ray accelerators hit their ceiling. A whole family of mass-dependent models is ruled out.
The hunt narrows from here. Theorists modeling supernova remnants, pulsars, and the cosmic neighborhood around our solar system now have a sharper target to fit.
The next generation of detectors will know exactly where in the spectrum to look – and which kind of source signature should match.
The study is published in Nature.
—–
Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.
Check us out on EarthSnap , a free app brought to you by Eric Ralls and Earth.com.
—–