![]() | Annu. Rev. Astron. Astrophys. 2002. 40:
319-348 Copyright © 2002 by Annual Reviews. All rights reserved |
Greisen (1966)
and
Zatsepin &
Kuzmin (1966)
pointed out that EeV cosmic
rays lose energy due to photo-pion production through interaction
with the CMB. These losses limit the propagation distance
for 1020 eV particles to about
60 Mpc. Yet no clear correlation has been
found between the arrival direction of high energy cosmic rays and
the most likely sites of origin for EeV particles, namely, AGN
at distances less than 60 Mpc
(Elbert &
Sommers 1995,
Biermann 1999).
One solution to the GZK paradox is to assume the energetic particles
are isotropized in the IGM by tangled magnetic fields, effectively
randomizing their observed arrival direction. Such isotropization
requires fields in the local super-cluster
0.3 µG
(Farrar 2000,
Isola, Lemoine,
& Sigl 2001).