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

TL;DR

This paper explores how magnetic lensing in galaxy clusters, specifically Virgo, influences ultrahigh energy cosmic ray observations, revealing significant anisotropies and biases in interpreting cosmic ray data.

Contribution

It demonstrates the impact of cluster magnetic fields on UHECR flux anisotropy and energy spectrum, highlighting the importance of magnetic lensing effects in data analysis.

Findings

Strong anisotropy in UHECR flux if M87 is the sole source

Energy-dependent lensing causes significant spectrum variation among observers

Anisotropies can bias source density and spectrum estimations

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.