Deflection of Ultra–High‐Energy Cosmic Rays by the Galactic Magnetic Field: From the Sources to the Detector

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Abstract
We report the results of three-dimensional simulations of the trajectories of ultra-high-energy (UHE) protons and Fe nuclei (with energies E = 4 × 1019 and 2.5 × 1020 eV), propagating through the Galactic magnetic field (GMF) from the sources to the detector. A uniform distribution of antiparticles is backtracked from the detector, at the Earth, to the halo of the Galaxy. We assume an axisymmetric, large-scale spiral magnetic field that permeates both the disk and the halo. A normal field component to the Galactic plane (Bz) is also included in part of the simulations. We find that the presence of a large-scale GMF does not generally affect the arrival directions of the protons, although the inclusion of a Bz component may cause significant deflection of the lower energy protons (E = 4 × 1019 eV). Error boxes larger than or equal to ~5° are most expected in this case. On the other hand, in the case of heavy nuclei, the arrival direction of the particles is strongly dependent on the coordinates of the particle source. The deflection may be high enough (>20°) as to make any identification of the sources extremely difficult unless the real magnetic field configuration is determined accurately. Moreover, not every incoming particle direction is allowed between a given source and the detector. This generates sky patches that are virtually unobservable from the Earth. In the particular case of the UHE events of Yakutsk, Fly's Eye, and Akeno, they come from locations for which the deflection caused by the assumed magnetic field is not significant.
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