Cosmic rays are atomic nuclei accelerated to near-light speeds. They arrive at Earth from all directions, and their composition tells us what's happening in the far reaches of the Galaxy. The NUCLEON satellite spent several years collecting these particles, and now its data reveal a strange pattern: the rarer a nucleus is in nature, the harder its spectrum.

A spectrum is the relationship between the number of particles and their energy (or rigidity — energy divided by charge). Usually, as particle energy increases, their number decreases, and the slope of this relationship is the same for all nuclei. But for rare odd nuclei (like fluorine or sodium), the slope turned out to be different: their fraction among high-energy particles is higher than that of their abundant even neighbors (oxygen, neon). It's as if odd nuclei are better at being accelerated.

The authors of the study tried to explain this through incomplete ionization. When a supernova shock wave accelerates matter, atoms may not lose all their electrons. A nucleus with remaining electrons has a smaller effective charge, and thus, at the same energy, a higher rigidity. Such "half-dressed" nuclei might slip through magnetic barriers differently than fully ionized ones. If rare odd nuclei more often retain electrons in the early stages of acceleration, their spectrum would be harder.

For now, this is a hypothesis, but it elegantly links NUCLEON's data with shock-wave physics. If confirmed, we could better understand how supernovae stamp out cosmic rays, and why the composition of these rays differs from that of ordinary matter.