Constraining the Extragalactic Magnetic Field: Auger Data Meet UHECR Propagation Modeling

Recent analyses from the Pierre Auger Collaboration suggest correlations between the arrival directions of Ultra-High-Energy Cosmic Rays (UHECRs) and catalogs of starburst galaxies (SGBs) 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 catalog correlations, jetted AGNs are dominated by CentaurusA when accounting for source distances and deflections. Using our framework, we derive 90% confidence level upper limits on the local EGMF strength of 4.4nGMpc$^{1/2}$ for SBGs and 6.7nGMpc$^{1/2}$ for jetted AGNs. Assuming instead that the UHECR deflections predominantly arise from the Galactic magnetic field (GMF), we obtain a GMF upper limit of $1.4 , μ$Gkpc$^{1/2}$ for a Galactic halo size of 30~kpc.

💡 Research Summary

This paper presents a novel analysis aimed at constraining the properties of the extragalactic magnetic field (EGMF) using data from the Pierre Auger Observatory on Ultra-High-Energy Cosmic Rays (UHECRs). The study revisits previously reported correlations between UHECR arrival directions and catalogs of starburst galaxies (SBGs) and jetted active galactic nuclei (AGNs), but with a critical advancement: it explicitly models the deflections UHECRs experience while propagating through turbulent EGMFs, rather than using a simplified, distance-independent smearing angle.

The authors adopt the same UHECR dataset, source catalogs, and maximum-likelihood statistical framework used by the Auger Collaboration. However, they replace the empirical Fisher distribution smearing with physics-based simulations. Using the CRPropa 3 propagation code, they simulate the trajectories of UHECRs from each cataloged source through a purely turbulent magnetic field with a Kolmogorov power spectrum. This allows them to generate probability sky maps that inherently depend on the EGMF root-mean-square strength (B_rms), the source distances, and the UHECR energies. The model has two key parameters: the signal fraction (α), representing the proportion of events originating from the catalog, and the EGMF strength.

The results yield two major insights. First, while the correlation signal for SBGs is dominated by the same sources as in the standard Auger analysis even when deflections are included, the picture changes for jetted AGNs. When accounting for distances and magnetic deflections, the contribution from Centaurus A becomes overwhelmingly dominant, suggesting this catalog’s correlation may be driven primarily by this single, nearby source. Second, by fitting their model to the Auger data, the authors derive upper limits on the local EGMF strength. Expressed in terms of the effective parameter ˜B = B_rms √(l_coh), where l_coh is the coherence length, they find 90% confidence level upper limits of 4.4 nG Mpc^1/2 for the SBG catalog and 6.7 nG Mpc^1/2 for the jetted AGN catalog. Additionally, under an alternative hypothesis where deflections are assumed to occur predominantly in the Galactic magnetic field (GMF), they obtain an upper limit of 1.4 μG kpc^1/2 for a Galactic halo size of 30 kpc.

In conclusion, this work demonstrates a powerful methodology to use observed UHECR anisotropies not just to identify potential sources, but to inversely constrain the intervening magnetic fields. The derived limits provide valuable input for models of cosmic ray propagation and magnetogenesis in the universe. The findings underscore the importance of incorporating detailed propagation physics, including source distances and structured magnetic fields, when interpreting UHECR arrival directions. The study paves the way for more comprehensive future analyses that could simultaneously fit UHECR spectrum, composition, and anisotropy data within a unified propagation framework.