Cosmic-ray energy spectrum and composition up to the ankle: the case for a second Galactic component

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
e-mail: satyendra.thoudam@lnu.se
² Department of Physics and Electrical Engineering, Linnéuniversitetet, 35195 Växjö, Sweden
³ Astronomical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
⁴ NIKHEF, Science Park Amsterdam, 1098 XG Amsterdam, The Netherlands
⁵ ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands

Received: 10 May 2016
Accepted: 17 July 2016

Abstract

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 10¹⁸ eV. 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 ~ 10¹⁴ eV and the “knee” in the all-particle spectrum, cannot explain the observed all-particle spectrum above ~ 2 × 10¹⁶ eV. 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 ~ 10¹⁸ eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. In this two-component Galactic model, the knee at ~ 3 × 10¹⁵ eV and the “second knee” at ~ 10¹⁷ eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. 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 × 10¹⁸ eV), and find that extra-galactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. We also provide arguments that an extra-galactic contribution is unlikely to dominate at or below the second knee. 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. 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 ~ 10¹⁷ and 10¹⁸ eV.