An upper limit on the strength of the extragalactic magnetic field from ultra-high-energy cosmic-ray anisotropy

TL;DR

This paper establishes an upper limit on the extragalactic magnetic field strength based on ultra-high-energy cosmic-ray anisotropy observations, refining previous constraints especially at larger coherence lengths.

Contribution

It provides new upper bounds on the extragalactic magnetic field strength derived from cosmic-ray anisotropy data, improving existing limits at larger scales.

Findings

Upper limit B < 0.7-2.2 x 10^-9 G for lambda_B < 100 Mpc

Limit B < 0.7-2.2 x 10^-10 G at larger scales

Results are constrained by unknown cosmic-ray composition

Abstract

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.