Pinpointing a Source of PeV Cosmic Rays
The cosmic-ray spectrum measured at facilities such as the Large High Altitude Air Shower Observatory (LHAASO) in China plummets as particle energies rise. Up to 3–4 peta-electron-volts (PeV or 1015 eV), the flux–energy relationship approximately follows a power law. Beyond that point, the flux drops off even more steeply. Supernovae are expected to dominate the lower end of the spectrum, and physicists have proposed that the change in slope—the “knee”—marks the energy limit for cosmic rays accelerated by those events. But specific supernovae associated with cosmic rays that go all the way up to the knee have been difficult to identify. Now the LHAASO team has detected cosmic rays with energies of at least 0.3 PeV from the ancient but still expanding supernova IC 443 [1].
Because cosmic rays are charged, their trajectories are distorted by galactic magnetic fields, which scramble information about the rays’ origins. The LHAASO researchers observed IC 443 in gamma rays, which propagate directly from their source.
Gamma rays emanating from IC 443 could conceivably originate from two kinds of particles accelerated to relativistic energies by the supernova’s shockwave. Electrons could interact with photons in ambient starlight and the cosmic microwave background, boosting those photons to gamma-ray energies. Alternatively, protons, which make up most cosmic rays, could strike particles in the dense molecular cloud that neighbors IC 443. Those collisions would create neutral pions, which quickly decay into gamma rays and other particles.
The LHAASO researchers found that the measured gamma-ray spectrum matches the pion-decay model, not the relativistic-electron one. Although the inferred cosmic-ray energies are still an order of magnitude short of the knee, the spectrum shows no sign of a cutoff beyond 0.3 PeV.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- Z. Cao et al. (LHAASO Collaboration), “Evidence of cosmic-ray acceleration up to sub-PeV energies in the supernova remnant IC 443,” Phys. Rev. Lett. 136, 161002 (2026).