Deflections of UHECRs in the Galactic magnetic field

• Published in: Journal of cosmology and astroparticle physics Vol. 2019; no. 5; p. 4
• Main Authors: Farrar, Glennys R., Sutherland, Michael S.
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 03.05.2019
• Subjects: Anisotropy; Augers; Celestial sphere; Centroids; Coherence length; Dipoles; Fields (mathematics); Interstellar magnetic fields; Magnetic fields; Outliers (statistics); Rigidity
• ISSN: 1475-7516; 1475-7516
• DOI: 10.1088/1475-7516/2019/05/004

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≥1018V, to describe Fe in the UHE energy range above ∼55 EeV, 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 a large scale coherent and a structured random component, and determine deflections for fifteen rigidities from 1018.0 to 1020.0 V, for two different coherence length choices for the random component, Lcoh=30 pc and 100 pc. We also 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 >5×107 isotropically-distributed anti-CRs of the given rigidity, which we backtrack using the code CRT. We present skyplots and tables characterizing the arrival directions, for representative 1o 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 90ˆ. 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. One empirical regularity is that the average deflection of UHECRs from a given source, while growing with decreasing rigidity and depending on source direction and Lcoh, is about twice as large as the RMS angular spread about the mean arrival direction (centroid), independently of source direction, rigidity or Lcoh, albeit with outliers. 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.