Speaker
Description
Knowledge of the coherent Galactic magnetic field (GMF) is important for many purposes, especially for interpreting the arrival directions of ultra-high-energy cosmic rays (UHECRs). For more than a decade, the community has relied mainly on a single model of the coherent field (Jansson-Farrar, JF12). Recently, a suite of models has been developed (Unger-Farrar, UF23) based on the latest data and with improved analysis of the GMF, which for the first time provides a practical measure of the possible range of variations of the field. We calculate the large-scale flux distribution of UHECRs including deflections in these new GMF models, assuming that the sources follow the large-scale structure of the universe. We find that variations of the predicted dipole direction and amplitude due to uncertainties in the GMF are subdominant to cosmic variance from the discrete positioning of sources.
An important discovery of our work is the unanticipated sensitivity of the dipole amplitude to the coherent field model. The predicted dipole amplitude is significantly reduced for all UF23 models compared to the predecessor JF12 model. We trace this change to the accidental position of the peak of the extragalactic UHECR flux, which falls at the boundary of the strong flux de-magnification due to the GMF toward the central region of the Galaxy. This serendipitous sensitivity of UHECR anisotropies to the GMF model will be a powerful probe of the source distribution as well as Galactic and extragalactic magnetic fields. The demagnification by the GMF also impacts visibility of some popular source candidates.
