In the quiet harbor of the Local Bubble, a rarefied cavity of hot gas around the Sun, an explosion recently thundered. A supernova went off tens of parsecs away, and its echo still lingers in the neighborhood, reshaping the cosmic-ray landscape.

For a long time, the flux of particles from the Galaxy was thought to be uniform, like the light from many distant lamps. But anisotropy data, the unevenness of the flux across directions, refused to fit this picture. As particle energy increased, anisotropy grew faster than standard models predicted. The proton spectrum also behaved oddly: at 200 GeV it hardened, and above 50 TeV it softened again.

The key move is a small diffusion coefficient in the Local Bubble. Here, particles scatter in magnetic fields a thousand times slower than the galactic average. So the contribution from the nearby explosion is not smeared across the sky but remains a noticeable beam, which creates the observed anisotropy. The same source explains the spectral breaks: its particles add to the background, hardening the spectrum at 200 GeV, while above 50 TeV the source's flux fades: it hasn't had time to accumulate particles of such energies.

The model is testable. Future precise measurements of the spectrum and anisotropy in the 10–100 TeV range will show whether we truly live inside the echo of a recent explosion. For now, the picture is coming together: the quiet harbor wasn't so quiet after all, and the supernova's echo still shapes the particle flux we measure.