Galactic Magnetic Fields and the Origin of the Highest-Energy Cosmic Rays

by Michael Unger (KIT)

Thursday 29 Jan 2026, 15:00 → 16:30 Europe/Berlin

seminar room 3 (bldg. 1b)

Magnetic fields permeate galaxies with energy densities comparable to those of turbulence and thermal gas, influencing both galactic dynamics and the propagation of charged particles. Understanding the global structure of the Galactic magnetic field is therefore essential for interpreting a wide range of astrophysical observations, including ultra-high-energy cosmic rays (UHECRs). In this seminar, I will present recent advances in modeling the large-scale magnetic field of the Milky Way based on new full-sky data of extragalactic Faraday rotation measures and polarized synchrotron emission from WMAP and Planck. The analysis combines divergence-free parametric magnetic-field models with up-to-date descriptions of thermal and cosmic-ray electrons, tuned to pulsar dispersion measures and cosmic-ray observations. The result is a major revision of the widely used Jansson–Farrar Galactic magnetic field model, along with an ensemble of alternative models that quantify current systematic uncertainties. I will then discuss the implications of these models for ultra-high-energy cosmic rays observed by the Pierre Auger Observatory and the Telescope Array. Accounting for Galactic magnetic deflections is essential for interpreting the observed anisotropies and for tracing the highest-energy events back to their sources. Applying the new models shows that some of the best-localized events do not point to plausible steady accelerators, strengthening the case that at least a fraction of the highest-energy cosmic rays originate from transient sources.