Until a decade ago, the cosmic ray spectrum from ~ 10 GeV to ~ 10 11 GeV was seen as a power law with two main features: a steepening from a spectral index γ ≈ −2.7 to γ ≈ −3.1 at about 3 × 10 6 GeV, commonly called the “knee”, and a flattening back to γ ≈ −2.7 at about 4 × 10 9 GeV, consequently denoted as the “ankle”.
Key words: diffusion / stars: winds, outflows / ISM: supernova remnants / galaxies: ISM / cosmic rays This prediction is in agreement with new measurements from LOFAR and the Pierre Auger Observatory which indicate a strong light component and a rather low iron fraction between ~ 10 17 and 10 18 eV. Our main result is that the second Galactic component predicts a composition of Galactic cosmic rays at and above the second knee that largely consists of helium or a mixture of helium and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast to most common assumptions. We also provide arguments that an extra-galactic contribution is unlikely to dominate at or below the second knee. We also discuss several variations of the extra-galactic component, from a minimal contribution to scenarios with a significant component below the “ankle” (at ~ 4 × 10 18 eV), and find that extra-galactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. In this two-component Galactic model, the knee at ~ 3 × 10 15 eV and the “second knee” at ~ 10 17 eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. We discuss two approaches for a second component of Galactic cosmic rays – re-acceleration at a Galactic wind termination shock, and supernova explosions of Wolf-Rayet stars, and show that the latter scenario can explain almost all observed features in the all-particle spectrum and the composition up to ~ 10 18 eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of different elements, optimised to explain the observed elemental spectra below ~ 10 14 eV and the “knee” in the all-particle spectrum, cannot explain the observed all-particle spectrum above ~ 2 × 10 16 eV. Motivated by the recent high-precision measurements of cosmic rays by several new-generation experiments, we have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to about 10 18 eV. Hörandel 1 ,4ġ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The NetherlandsĮ-mail: Department of Physics and Electrical Engineering, Linnéuniversitetet, 35195 Växjö, Swedenģ Astronomical Institute, Vrije Universiteit Brussel, PleinlBrussels, BelgiumĤ NIKHEF, Science Park Amsterdam, 1098 XG Amsterdam, The Netherlandsĥ ASTRON, PostAA Dwingeloo, The Netherlands Astronomical objects: linking to databases.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes
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