Cosmic rays misbehave in space station experiment

A new census of charged particles buzzing through space includes a puzzling feature that challenges predictions about how these particles originate. The results, presented April 15 at a conference in Geneva, may force scientists to rethink theories that focus on supernovas as the producers of these speedy particles.

Installed on the International Space Station in 2011, the Alpha Magnetic Spectrometer collects and identifies cosmic rays, charged subatomic particles that permeate the galaxy (SN: 3/21/15, p. 22). Based on the previously measured concentrations of galactic cosmic rays, many scientists suspect that the particles get flung toward Earth in the shock waves of exploding stars. But the new analysis of 300 million protons and 50 million helium nuclei adds a wrinkle to the shock wave explanation. While the number of particles observed generally drops steadily as their energy increases, at an energy of several hundred billion electron volts, the rate of that drop abruptly decreases. The shock wave scenario doesn’t support that sudden rate change, says University of Wisconsin–Madison particle astrophysicist Francis Halzen, who attended the cosmic ray–focused meeting.

The measurement, which confirms less precise findings from previous experiments, suggests an additional source of cosmic rays. “This structure really challenges our notions about the origin of galactic cosmic rays,” Halzen says.