How isotropic can the UHECR flux be?

Abstract

Modern observatories of ultra-high energy cosmic rays (UHECR) have collected over 10^4 events with energies above 10 EeV, whose arrival directions appear to be nearly isotropically distributed. On the other hand, the distribution of matter in the nearby Universe -- and, therefore, presumably also that of UHECR sources -- is not homogeneous. This is expected to leave an imprint on the angular distribution of UHECR arrival directions, though deflections by cosmic magnetic fields can confound the picture. In this work, we investigate quantitatively this apparent inconsistency. To this end we study observables sensitive to UHECR source inhomogeneities but robust to uncertainties on magnetic fields and the UHECR mass composition. We show, in a rather model-independent way, that if the source distribution tracks the overall matter distribution, the arrival directions at energies above 30 EeV should exhibit a sizeable dipole and quadrupole anisotropy, detectable by UHECR observatories in the very near future. Were it not the case, one would have to seriously reconsider the present understanding of cosmic magnetic fields and/or the UHECR composition. Also, we show that the lack of a strong quadrupole moment above 10 EeV in the current data already disfavours a pure proton composition, and that in the very near future measurements of the dipole and quadrupole moment above 60 EeV will be able to provide evidence about the UHECR mass composition at those energies.

9 pages, 8 figures; accepted version

References in corpus (6)

• Indications of Intermediate-Scale Anisotropy of Cosmic Rays with Energy Greater Than 57 EeV in the Northern Sky Measured with the Surface Detector of the Telescope Array Experiment
• Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory
• Magnetic Fields in the Milky Way
• Searches for Large-Scale Anisotropy in the Arrival Directions of Cosmic Rays Detected above Energy of $10^{19}$ eV at the Pierre Auger Observatory and the Telescope Array
• SimProp v2r4: Monte Carlo simulation code for UHECR propagation
• Full sky harmonic analysis hints at large UHECR deflections

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• Search for Large-scale Anisotropy on Arrival Directions of Ultra-high-energy Cosmic Rays Observed with the Telescope Array Experiment
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• Covering the celestial sphere at ultra-high energies
• Full-sky searches for anisotropies in UHECR arrival directions with the Pierre Auger Observatory and the Telescope Array
• Large-scale cosmic ray anisotropies with 19 years of data from the Pierre Auger Observatory
• Detecting ultra-high energy cosmic ray anisotropies through cross-correlations
• 2022 report from the Auger-TA working group on UHECR arrival directions
• Measurements and implications of cosmic ray anisotropies from TeV to trans-EeV energies
• Using Deep Learning to Enhance Event Geometry Reconstruction for the Telescope Array Surface Detector
• Feasibility of Correlated Extensive Air Shower Detection with a Distributed Cosmic Ray Network
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• Ultra-high-energy Cosmic Ray Sources can be Gamma-ray Dim
• Future ground arrays for ultrahigh-energy cosmic rays: recent updates and perspectives