Comment on “ Violation of the Greisen - Zatsepin - Kuzmin Cutoff : A Tempest in a ( Magnetic ) Teapot ? Why Cosmic Ray Energies above 10 20 eV May Not Require New

In a recent letter [1] with the same title, Farrar and Piran offered an explanation for the near isotropy of the arrival directions [2] of ultrahigh energy cosmic rays (UHECRs) and the apparent absence [3] of the so called ‘GZK cutoff’ in their spectrum around 10 eV due to pion photoproduction on the cosmic background radiation (CMB) that was predicted independently by Greisen [4] and by Zatsepin and Kuz’min [5]. They suggested that the extragalactic magnetic fields near the Milky Way are strong enough to deflect and isotropise the arrival directions of the UHECRs from a few nearby sources for which their travel time to Earth is shorter than their attenuation time by pion photoproduction on the CMB. They also estimated that this allows active galactic nuclei (AGNs) or gamma ray bursts (GRBs) to be the source of the UHECRs. However, these suggestions are inconsistent with various observations: a. If our Milky Way galaxy was embedded in a ∼ μG magnetic halo/slab of the local supercluster (LSC) with dimensions larger than the Larmor radius (eq. 5 of ref. [1]) of UHECRs, then ∼ GeV cosmic rays would have been confined magnetically for times exceeding the Hubble time. This contradicts the much smaller ‘age’ of the bulk of cosmic rays in the Milky Way that was inferred [6] from the measured isotopic ratios Al/Al and Be/Be. b. If our Milky Way galaxy was embedded in a ∼ μG magnetic halo/slab of Mpc dimensions, diffusion in this halo/slab would have produced a quasi uniform distribution of the bulk of the cosmic rays inside it. This contradicts the much smaller upper bounds on the densities of cosmic rays at the small and large Magellanic clouds (SMC and LMC,