An upper limit on the strength of the extragalactic magnetic field from
  ultra-high-energy cosmic-ray anisotropy
  If ultra-high-energy cosmic rays originate from extragalactic sources, the
offsets of their arrival directions from these sources imply an upper limit on
the strength of the extragalactic magnetic field. The Pierre Auger
Collaboration has recently reported that anisotropy in the arrival directions
of cosmic rays is correlated with several types of extragalactic objects. If
these cosmic rays originate from these objects, they imply a limit on the
extragalactic magnetic field strength of B < 0.7-2.2 x 10^-9 (lambda_B / 1
Mpc)^-1/2 G for coherence lengths lambda_B < 100 Mpc and B < 0.7-2.2 x 10^-10 G
at larger scales. This is comparable to existing upper limits at lambda_B = 1
Mpc, and improves on them by a factor 4-12 at larger scales. The principal
source of uncertainty in our results is the unknown cosmic-ray composition.
