Deflections of UHECRs in the Galactic magnetic field

TL;DR

This study models ultra-high-energy cosmic ray deflections in the Galactic magnetic field using a realistic model, revealing complex patterns, multiple images, and significant deflections especially at low rigidities, impacting source identification.

Contribution

It provides the first high-resolution, comprehensive analysis of UHECR deflections with a realistic Galactic magnetic field model across a wide rigidity range.

Findings

Deflections increase rapidly below 10 EV rigidity.

Multiple images are common, especially at low rigidity.

Large regions of the sky are obscured in UHECRs below 10 EV.

Abstract

We report results of the first comprehensive, high resolution study of the deflections of UHECRs using a realistic model of the Galactic magnetic field and extending to sufficiently low rigidities, R = E/Z = 10^18 V, to describe Fe in the UHE energy range or the mixed composition reported by the Pierre Auger Collaboration at the energy of the observed dipole anisotropy. We use the Jansson-Farrar (JF12) model, which has both large scale coherent and a structured random components, and determine deflections for fifteen rigidities from 10^18.0 to 10^20.0 V, for L_coh = 30 pc and 100 pc. We check the sensitivity of UHECR deflections to the particular realization of the random field, for 3 rigidity values. For each rigidity and field model studied, the UHECR arrival direction distribution is determined for an arbitrary source direction, by inverting the trajectories of > 5 x 10^7 isotropic anti-CRs of the given rigidity. We present skyplots and tables characterizing the arrival directions, for representative 1 deg sources. Except at high rigidity, the pattern of multiple images is very complex and depends strongly on the coherence length and source direction. For almost all sources, average deflections grow rapidly as the rigidity falls below 10 EV and deflections commonly are greater than 90 deg. Magnification and demagnification can be strong at almost all rigidities, and varies significantly with source direction. Much of the extragalactic celestial sphere, behind and south of the Galactic Center, cannot be seen in UHECRs below 10 EV. The pattern of deflections obtained with the coherent field alone can be significantly different than with the complete field model, even as regards the position of the centroid, for some source directions and rigidities. Multiple images, sometimes very widely separated, are common for small coherence length, especially for low rigidity.