Why does a giant star cluster's particle accelerator produce such extreme gamma rays?

Massive star clusters accelerate particles to extreme energies, and Cygnus OB2 recently revealed gamma-ray emission reaching petaelectronvolt scales—far higher than expected. By modeling how accelerated particles diffuse through the bubble surrounding the cluster and interact with gas, the authors show that standard particle transport fails; the diffusion coefficient must be dramatically reduced over a 150-parsec region to explain both the spectrum and shape of observed emission. This suggests either extreme magnetic conditions in the cluster wind or a missing high-energy source.