Figure 1
Illustration: Alan Stonebraker

Figure 1: The overall cosmic-ray spectrum showing the average slope of the spectrum over three important intervals: below the “knee” at 1015 eV, from the “knee” to the “ankle” just below 1019 eV, and around the predicted GZK cutoff near 1019–1020 eV. (Lower inset) The high-energy data measured at the Auger telescope (the white triangles indicate upper limits only) [4]. Blue lines are guides to the eye, drawn using the spectral indices from Ref. [4] (Auger) above the ankle, and the spectral index from Ref. [3] (HiRes) below the ankle. The upper inset sketches the interaction between a cosmic-ray particle (proton or neutron) and a photon, γ, in the cosmic microwave background that results in the photoproduction of secondary π mesons via a Δ resonance.