UHECR observations and lensing in the magnetic field of the Virgo cluster

Abstract

We discuss how lensing by magnetic fields in galaxy clusters affects ultrahigh energy cosmic ray (UHECR) observations. As specific example, we use Virgo together with the cluster magnetic fields obtained earlier in a constrained simulation of structure formation including MHD processes. We find that, if M87 is the single source of UHECRs from Virgo, the emitted flux is strongly anisotropic in the most interesting energy range, (50-100)EeV, and differs from the average value by a factor five or more for a significant fraction of observers. Since magnetic lensing is energy dependent, the external energy spectrum as seen by different observers varies strongly too. These anisotropies are averaged out in the case that all active galactic nuclei in Virgo emit UHECRs. In both cases, the anisotropies of the emitted UHECR flux may introduce an important bias in the interpretation of UHECR data like, e.g., the determination of the source density n_s and the source energy spectrum of UHECRs.Comment: 12 pages, 15 eps figures; v2: extended discussion of modifications in external energy spectrum, matches version to be publishe