Cosmic rays seen at Earth show a wide range of particle energies, from 107 electron-volts (eV) to more than 1020 eV, the latter being about the same as the kinetic energy of a 450 gram football (soccer ball) being kicked across the pitch at about 8 meters per second. A plot of cosmic ray energies from the Milky Way galaxy often shows a fair amount of what scientists might call “structure”—interesting deviations from the underlying trend called “knees” and “ankles” that indicate new processes or methods of cosmic ray production taking place at that energy.
Researchers have proposed that regions along the curve where the slope changes—the “knees”—signify the energy limit for cosmic rays accelerated by the material the exploded star sloughs off. But it has proven difficult to examine specific supernovae associated with cosmic rays at energies up to the knee.
Moreover, the origins of cosmic rays seen on Earth are not always clear. Cosmic ray particles sometimes have an electrical charge, so their path to Earth is distorted by magnetic fields in the galaxy. This reduces useful information about the origin of these rays.
New clues from gamma-ray observations
Now a large group from the LHAASO collaboration in China have measured the spectrum of the high-energy gamma rays from a particular supernova remnant and found it matches a decay model of pions created in the molecular cloud that surrounds the supernova remnant, which themselves were created when supernova protons interact and scatter with a surrounding molecular cloud.
Their work has been published in Physical Review Letters.
The group consists of several hundred researchers/co-authors who are members of the LHAASO Collaboration at the Large High Altitude Air Shower Observatory in southwestern China. Using the ground-based observatory, they observed emissions of high-energy gamma rays, which are photons of very high energy, from the supernova remnant of IC 443, a star 5,000 light years from Earth in the constellation Gemini that exploded about 30,000 years ago.
It’s also known as the Jellyfish Nebula and is still expanding. It is one of the best-studied cases of remnants around exploded supernovae interacting with surrounding molecular clouds.
How supernova remnants create shocks
The explosion of a star starts deep inside it. The explosion wave eventually reaches the outside surface layers of the star, and only then does an optical (visible) outburst take place.
The supernova can expel huge masses of material—many times the mass of our sun—at high speeds of several percent of the speed of light, which drives shock waves into the interstellar medium, sweeping up gas and dust. Such a material conglomerate is called a “supernova remnant,” and the interaction between the shock waves and the supernova remnants plays a key role in how the remnants evolve and emit radiation that can become the cosmic rays seen, in this case, many millennia later.
These shocks can also function as cosmic ray accelerators when the cosmic ray particles are hadrons, such as protons or pions.
Two competing origins for gamma rays
The Collaboration viewed the IC 443 remnants via gamma rays, which have no electric charge and thus arrive at Earth directly from the supernova cloud, unaffected by the galaxy’s magnetic fields.
There are two possible ways that cosmic rays from a supernova could originate and propagate, and the LHAASO Collaboration wanted to determine which was involved with the IC 443 cosmic rays.
One way has highly energetic, relativistic electrons accelerated by the supernova’s shockwaves, then interacting with other photons in the vicinity, present from either ambient starlight or the cosmic microwave background and boosting these photons to gamma ray status.
Another method is that protons could collide with particles in the dense molecular cloud near IC 443, with these collisions creating electrically neutral pions, which quickly decay into gamma rays and other particles.
Evidence for pion decay and protons
The energy spectrum measured by the LHAASO researchers—the probability of a given cosmic ray energy as a function of its energy—made it clear that the measured gamma-ray spectrum matched the neutral pion-decay model, appearing as a bump in the gamma ray spectrum, not the highly energetic relativistic-electron scenario.
The cosmic ray energies were still an order of magnitude below the known knee on the spectrum curve. The spectrum gave no signal of a particle cutoff beyond 0.3 PeV, which provided good evidence that the remnant shocks can accelerate protons to sub-PeV energy levels.
The Collaboration’s finding strengthens “the evidence for the hypothesis that [supernova remnants] are one class of sources of galactic cosmic rays,” they wrote.
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Publication details
Zhen Cao et al, Evidence of Cosmic-Ray Acceleration up to Sub-PeV Energies in the Supernova Remnant IC 443, Physical Review Letters (2026). DOI: 10.1103/pxn6-qzhz. On arXiv: DOI: 10.48550/arxiv.2510.26112
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Evidence of cosmic-ray acceleration from a nearby supernova remnant (2026, May 30)
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