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 (EGMF). 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 EGMF strength of B < 0.7– G for coherence lengths λ B < 100 Mpc and B < 0.7–2.2 × 10−10 G at larger scales. This is comparable to existing upper limits at λ B = 1 Mpc, and improves on them by a factor of 4–12 at larger scales. The principal source of uncertainty in our results is the unknown cosmic-ray composition.