Origin and Composition of Ultrahigh Energy Cosmic Rays

Abstract

DISCUSSIONS of the problem of the origin of ultrahigh energy cosmic rays have centred on three characteristics of their energy spectrum: (1) an apparent steepening of the power-law integral spectrum, E−γ from γ≃ 1.7 to γ≃ 2.2 at an energy of ∼ 1015 eV ; (2) an apparent flattening of the spectrum at an energy above 3×1018 eV where γ≃1.6 and (3) the apparent absence of a cutoff in the energy spectrum of air-shower events at energies above 1019 eV. Recently, the validity of the second characteristic has been questioned and a new analysis of both the Haverah Park and Volcano Ranch air-shower data has indicated that γ≃2.2 for energies above 1017 eV (refs. 1 and 2). I suggest that, if this is the case, the absence of a flattening above 3 × 1018 eV may have implications for the debate about the origin of ultrahigh energy cosmic rays (UECR).