Galactic magnetic field bias on inferences from UHECR data

Abstract

A consequence of Liouville's theorem indicates that the recently observed large scale anisotropy in the arrival direction of Ultra-High-Energy Cosmic Rays (UHECRs) cannot be produced from an isotropic extragalactic flux, thus some anisotropy already needs to be present outside our Galaxy. But in this case, the observed spectrum and composition of UHECRs differ from the one outside of the Milky Way, due to the suppression or the amplification of the UHECR flux from certain directions by the Galactic magnetic field. In this work, we investigate this effect for the case of a dipole and a quadrupole anisotropy, respectively, for the widely-used JF12 and PT11 magnetic field models. We investigate bounds on the maximal amplitude of the observed anisotropy and the maximal charge number of UHECRs. Furthermore, the flux modification is discussed in the light of the recent Auger results about the dipolar anisotropy and the mass composition. We find that this effect yields a modification of the observed flux of up to ∼ 25% for the investigated magnetic field model and the observed dipole, in particular for a heavy chemical composition of UHECRs as suggested by Auger data interpreted assuming the `Sibyll2.3c' or the `EPOS-LHC' model of hadronic interactions in air showers.