DAMPE observes charge-dependent limit of cosmic ray acceleration

Cosmic rays (CRs) are energetic particles traveling through space at speeds close to that of light. They consist of atomic nuclei, electrons, positrons, high-energy gamma rays, and neutrinos, and are thought to originate from extreme astrophysical objects such as supernova remnants, rapidly rotating neutron stars, and accreting black holes.

Although CRs have been studied for more than a century, fundamental questions—how and where they are produced, how they are accelerated, and how they propagate and interact in interstellar space—have persisted.

Answering these questions relies on understanding the energy spectrum of CRs—how particle numbers vary with energy. For this reason, obtaining precise measurements of the spectra of individual CR components is key to understanding cosmic-ray physics.

Now, the international collaboration of the DArk Matter Particle Explorer (DAMPE) satellite has, for the first time, directly observed the charge-dependent "spectral softening" of five primary cosmic-ray nuclei, i.e. protons, helium, carbon, oxygen, and iron, whereby the number of these particles drops off more steeply at higher energies.

This observation confirms the charge-dependent acceleration model—termed the Peters cycle—which was first proposed in 1961 by Bernard Peters. According to the model, the maximum energy a cosmic ray can reach is proportional to its electric charge (Z). It also offers a key clue to solving the longstanding mystery of the origin of Galactic cosmic rays.

The study was published in Nature on April 29. It was led by researchers from the Purple Mountain Observatory of the Chinese Academy of Sciences, along with their collaborators from other institutions.

DAMPE, also known as "Wukong," is designed to study high-energy CRs and indirectly probe dark matter. Since its launch in late 2015, DAMPE has operated flawlessly and has recorded about 18.5 billion high-energy particle events. Its excellent energy resolution, good particle identification capability, and reasonably large acceptance (how many particles can be collected) make it suitable for studying the spectral structures of CRs, particularly in the tera- to peta-electronvolt range.

Based on nine years of data collected in orbit, the DAMPE collaboration precisely measured the spectra of the five most abundant cosmic-ray nuclei and, for the first time, directly detected distinct spectral softenings in carbon, oxygen, and iron nuclei by extending the measurements to the peta-electronvolt energy range. Combined with the updated proton and helium spectra, the collaboration found that spectral softening appears universally at a rigidity (momentum per unit charge) of about 15 teravolts, and nuclei-mass-dependent softening is rejected at a confidence level of >99.999%.

These findings, combined with large-scale anisotropy measurements, indicate the presence of a nearby cosmic-ray accelerator, with the observed spectral softening marking its charge-dependent energy limit.

The DAMPE observation provides the first experimental verification of the Peters cycle, which posited that particle acceleration in magnetic fields should obey a charge-dependent limit. With this achievement, DAMPE is expected to shed new light on fundamental questions about cosmic-ray physics.