Constraining the extragalactic magnetic field: Auger data meet UHECR propagation modelling

ABSTRACT Recent analyses from the Pierre Auger Collaboration suggest correlations between the arrival directions of ultra-high-energy cosmic rays (UHECRs) and catalogues of starburst galaxies (SBGs) and jetted active galactic nuclei (AGNs). We revisit these analyses using the same methodology as Auger, but explicitly incorporating UHECR deflections in turbulent extragalactic magnetic fields (EGMFs). We demonstrate that while for SBGs the same sources as for the generic Auger analysis dominate the catalogue correlations, jetted AGNs are dominated by Centaurus A when accounting for source distances and deflections. Using our framework, we derive 90 per cent confidence level upper limits on the local EGMF strength of 4.4 nG Mpc$^{1/2}$ for SBGs and 6.7 nG Mpc$^{1/2}$ for jetted AGNs. Assuming instead that the UHECR deflections predominantly arise from the Galactic magnetic field (GMF), we obtain a GMF upper limit of $1.4 \, \mu$G kpc$^{1/2}$ for a Galactic halo size of 30 kpc.