The propagation of UHE cosmic rays Academic Article uri icon

abstract

  • We study the propagation of ultra-high-energy cosmic rays (UHECR) in the Galaxy, concentrat-ing on the energy range below the ankle. The particles above the ankle are generally believed to be extragalactic, showing both a flattening of slope in the observed flux, and little anisotropy below the GZK cutoff, and some anisotropy towards the local supercluster above it. The particles below the ankle could well be galactic in origin, but the viability of this scenario has not been con-vincingly demonstrated to date. In this paper, we consider the hypothesis that UHECR production scales with star formation, which includes the hypothesis of UHECR origin from long GRB [13]. In an earlier publication we have already demonstrated that forward shocks from long GRB in the interstellar medium are energetically sufficient to produce the Galactic cosmic-ray component up to the ankle at 4 × 10 18 eV. Here we investigate in detail other observational constraints. A Monte-Carlo method based on analytical solutions to the time-dependent diffusion problem is used to account for intermittency. Assuming a source population similar to that of long GRB, we derive constraints arising from intermittency and the requirement to satisfy observational limits on the composition and anisotropy. It is shown that the composition and anisotropy at 10 18 eV are difficult to reproduce and require that either the particle mean free path is unusually small or that the composition is heavier than suggested by recent Auger data. We therefore consider it highly desirable that steps be taken to reduce the systematic uncertainty in the experimental derivation of the UHECR composition around 10 18 eV.

publication date

  • January 1, 2010