[Paper Review] What can be learnt from UHECR anisotropies observations? Paper I : large-scale anisotropies and composition features

D. Allard, J. Aublin+2 more|arXiv (Cornell University)|Oct 20, 2021

Astrophysics and Cosmic Phenomena11 references4 citations

TL;DR

This paper investigates how large-scale anisotropies in ultra-high-energy cosmic ray (UHECR) arrival directions—particularly dipole and quadrupole modulations—can constrain UHECR source models and composition. Using realistic simulations of UHECR propagation including energy losses, photo-dissociation, and magnetic deflections, it shows that while the observed dipole can be reproduced under various assumptions, the direction of the dipole is highly unnatural in most models, suggesting potential issues with source distribution or Galactic magnetic field assumptions. The results highlight the importance of future high-statistics observatories and composition-resolved anisotropy studies for breaking degeneracies in source scenario identification.

ABSTRACT

We investigate the implications of the current data regarding large scale anisotropies, and examine to what extent they can be used to constrain the origin of UHECRs and the astrophysical parameters of the source scenarios. We discuss the possibility of observing an associated anisotropy of the composition, the potential benefit of the separation of the different nuclear components and the interest of observing the UHECR sky with larger exposure future observatories. We simulate UHECR sky maps for various astrophysical scenarios satisfying the current observational constraints, taking into account the energy losses of the UHE protons and nuclei and their deflections by intervening magnetic fields. We investigate scenarios in which the UHECR source distribution follows that of the galaxies, varying the source composition and spectrum, the source density and the magnetic field models. We apply similar analyses as those used by the Auger collaboration. We find that: i) reproducing the observed dipole anisotropy and its energy evolution is relatively easy within our assumptions; ii) this agreement can be obtained with different sets of assumptions on the astrophysical parameters, and is thus not, at this stage, very constraining for UHECR source scenarios; iii) the actual reconstructed direction of the dipole appears non natural in essentially all scenarios investigated, and challenges their main assumptions on the source distribution or the assumed magnetic field configuration, especially in the Galaxy; iv) except for protons, the energy range in which the GZK horizon strongly reduces is a key target for anisotropy searches for each given nuclear species; v) The composition anisotropy naturally expected in our models is unlikely to account for that recently reported by Auger unless the observed amplitude is a strong positive statistical fluctuation of an intrinsically weaker signal.

Motivation & Objective

• To assess how large-scale UHECR anisotropies—especially dipole and quadrupole modulations—can constrain astrophysical source scenarios and UHECR composition.
• To evaluate the robustness of the observed dipole modulation in Auger data across diverse astrophysical and physical parameter sets.
• To investigate whether composition anisotropies, such as differences in mean mass across sky regions, can be naturally explained by propagation effects or require new source physics.
• To determine the potential of future high-exposure observatories in resolving degeneracies in UHECR source models through improved statistical power and composition separation.
• To examine the implications of energy-dependent dipole direction evolution for single-component vs. multi-component UHECR models.

Proposed method

• Simulating UHECR sky maps using a wide range of astrophysical scenarios, including galaxy-mapped source distributions with varying source densities, compositions (protons, nuclei), and source spectra.
• Incorporating physical effects such as photo-dissociation, energy losses (GZK effect), and deflections by extragalactic and Galactic magnetic fields (EGMF and GMF) with varying models.
• Generating 300 independent realizations per scenario to mimic statistical fluctuations and apply analysis techniques similar to those used by the Auger collaboration.
• Analyzing the dipole and quadrupole moments of the simulated arrival direction distributions as a function of energy, focusing on amplitude and direction evolution.
• Testing the detectability of composition anisotropies by comparing mean nuclear masses in high- and low-event-rate regions of the sky.
• Assessing the impact of isolating light nuclear components (e.g., protons and helium) on the significance of quadrupole signals and model constraints.

Experimental results

Research questions

• RQ1 Can the observed dipole modulation in UHECR arrival directions above 8 EeV be consistently reproduced across different source composition and magnetic field models?
• RQ2 Why does the reconstructed dipole direction in Auger data appear highly unnatural in most simulated scenarios, and what does this imply about the assumed source distribution or Galactic magnetic field?
• RQ3 To what extent can the energy evolution of the dipole direction serve as a discriminant between single-component and multi-component UHECR models?
• RQ4 What is the expected level of quadrupole modulation in future high-statistics UHECR datasets, and how does it depend on EGMF strength and coherence scale?
• RQ5 Can the recently reported composition anisotropy (lighter composition near the Galactic plane) be explained by propagation effects, or does it require a distinct astrophysical component?

Key findings

• The observed dipole modulation in Auger data can be reproduced in a wide range of astrophysical scenarios, indicating that the dipole alone cannot yet constrain source parameters uniquely.
• The direction of the reconstructed dipole in the highest-significance energy bin is highly unnatural in all simulated models, suggesting a need to reconsider assumptions about the Galactic magnetic field or source distribution.
• Energy-dependent evolution of the dipole direction, if confirmed with higher statistics, could become a powerful discriminant between single-component and multi-component UHECR models.
• Most simulated scenarios predict a significant quadrupolar modulation, especially when the light UHECR component is isolated, implying that future observations should search for such signals.
• The energy range where the GZK horizon strongly suppresses heavier nuclei is a key target for anisotropy studies, as it enhances sensitivity to source composition and distribution.
• The observed composition anisotropy reported by Auger is unlikely to be explained by propagation-induced composition differences unless it is a strong statistical fluctuation, suggesting a possible additional astrophysical component.