The most energetic cosmic ray electrons ever detected give us some important clues about the origin of these mysterious particles. Since 2015, more than 7 million particles have entered the CALorimetric Electron Telescope (CALET) attached to the International Space Station’s solid, broad spine.

This large collection has provided scientists with a robust data set that points to at least one nearby source of cosmic ray electrons and possibly more.

“The most exciting part is observing the highest energies,” says Nicholas Kennady, an astrophysicist at the University of Maryland in Baltimore and a member of the CALET collaboration led by astrophysicist Shoji Tori of Waseda University in Japan. “We have several candidates above 10 teraelectron volts, and if these are confirmed to be true electron events, that’s really good evidence for a nearby source.”

We’ve known about energetic cosmic rays since signs of their existence were first noticed more than a century ago, and all this time their source has remained largely a mystery.

These are tiny particles, mostly atomic nuclei, but also subnuclear particles like protons and electrons, that glide through the universe at close to the speed of light and have more power than these little things should have. But like most energetic things in the universe, scientists believe they come from energy sources. The most likely explanation so far is supernova remnants that accelerate particles into space, but there are other possible sources.

One of these is the theoretical destruction of dark matter, so physicists are trying to study it and understand its origin. However, it is a little difficult to detect them. Here on Earth, cosmic rays collide with atoms and molecules in the atmosphere, creating showers of particles; What we detect is not the cosmic ray itself, but the rain.

Some high-energy electron detectors also suffer from interference from fast protons, which distorts the results. However, the CALET space experiment allows direct detection of cosmic rays up to high energies. Previous work made it possible to detect cosmic rays up to 4.8 teraelectron volts. This study showed that the number of cosmic rays decreases as the energy level increases.

By using CALET’s large data pool with new processes to filter out errors caused by interference from protons, the team was able to detect electron cosmic rays up to 7.5 teraelectron volts, greatly increasing the data range. And interestingly, they found that the attenuation of cosmic rays did not decrease at higher energies. In any case, the opposite seems to be the case: At the highest energies, cosmic rays appear to intensify.

We don’t yet know where they come from, but since cosmic rays lose energy as they move through space, the high energies indicate they came relatively close to us.

There are actually supernova remnants nearby; their location and proximity are consistent with the detection of some of the highest energy cosmic rays detected in the data set. The team hopes that ongoing observations will further clarify the origin.

“These CALET observations raise the exciting possibility that some nearby supernova remnants can be measured on Earth,” says physicist T. Gregory Guzik of Louisiana State University, leader of the US branch of the CALET Collaboration.

“Continued CALET measurements throughout the lifetime of the International Space Station will help shed new light on the origin and transport of relativistic matter in our galaxy.” Source