What can be learnt from UHECR anisotropies observations Paper III: Update with new data and Galactic magnetic fields models

TL;DR

This work investigates whether ultra-high-energy cosmic-ray (UHECR) anisotropies can constrain the origin and distribution of sources given uncertainties in Galactic and extragalactic magnetic fields. By generating realistic skymaps with sources tracing the galaxy distribution and applying Auger-like analyses across multiple GMF models (UF23, XH24, KST24) and a hybrid local-universe source catalog, the authors assess large-scale and small-/intermediate-scale anisotropies. The main finding is that improved GMF models yield better alignment of the simulated dipole direction with Auger data, yet large cosmic variance and magnetic-field uncertainties still preclude strong constraints on source density, distribution biases, or individual source contributions. The study emphasizes that advancing the interpretation of UHECR anisotropies will require tighter GMF constraints, refined source modeling, and markedly larger experimental exposures from next-generation observatories.

Abstract

Context. At large angular scales, the Pierre Auger Observatory has reported a significant dipole modulation in right ascension, while at intermediate angular scales, localized flux excesses have been identified by both the Auger and Telescope Array collaborations. These observations were investigated in the first two papers of this series. Aims. We examine the implications of these anisotropy measurements and assess to what extent they can be used to constrain the origin of UHECRs and the astrophysical or physical parameters of viable source scenarios. Methods. As in the first two papers of this series, we generate realistic UHECR sky maps for a wide range of astrophysical models consistent with current spectral and composition constraints, assuming that UHECR sources trace the distribution of galaxies in the Universe. We update our previous studies by incorporating the most recent models of the Galactic magnetic field and apply the same large- and intermediate-scale anisotropy analyses as those used by the Auger Collaboration to simulated datasets with current experimental exposure. Results. The main novelty of this third paper is the improved compatibility between simulations and data, in particular regarding the reconstructed dipole direction, when using several of the recently proposed Galactic magnetic field models. Despite this progress, our main conclusions remain unchanged: although the observed anisotropies are compatible with an extragalactic origin of UHECRs, present data and magnetic-field uncertainties do not allow strong constraints to be placed on the nature, spatial distribution, or density of UHECR sources. Conclusions. Further progress in the interpretation of UHECR anisotropies will require improved constraints on cosmic magnetic fields, advances in source modeling, and significantly larger experimental exposures

Figures (18)

• Figure 1: Top-left to bottom-left : Magnification maps in Galactic coordinates for a 5 EV rigidity assuming various GMF models for the regular component coupled to the JF12+Planck turbulent component. The GMF models shown are (from left to right and top to bottom) : UF23 base, UF23 spur, UF23 (twistX), KST24 and XH24. The turbulent GMF coherence lengths assumed are $\lambda_{\rm c}$=50 pc for UF23 base, UF23 spur and XH24, $\lambda_{\rm c}$=100 pc for KST24 and $\lambda_{\rm c}$=200 pc for UF23 (twistX). Bottom-right : rigidity evolution of the magnification factor in the region of the Virgo cluster for various models of the GMF regular component (see legend) coupled to JF12+Planck model for the turbulent component.
• Figure 2: Full sky density maps of UHECR arrival directions in Galactic coordinates, the assumed source distribution is that of the mother catalog ($\rho\simeq8\times 10^{-3}\,\rm Mpc^{-3}$). The color scale is linear and the value 1 represent the average density calculated over the whole skymap. For all the maps a 1 nG EGMF with $\lambda_{\rm c}$=200 kpc is assumed, the various panels show the UHECR skymaps obtained for various assumptions on the GMF model, from left to right and from top to bottom : No GMF (the labels show various prominent galaxies and structures of the nearby Universe), UF23 base, UF23 spur, UF23 twistX, KST24 and XH24. The coherence lengths of the turbulent component of the GMF are the same as those mentioned in Fig. \ref{['Magnif']}.
• Figure 3: Location of the dipole reconstructed for each of the 300 different source distributions drawn from the mother catalog. The barycenter of the predictions is indicated with a large square marker. Auger data are shown with a large red circle marker surrounded by its 1 and 2$\sigma$ error ellipses. Each panel represents a different combination of assumed source distribution and GMF models (see legend). The map shown are in galactic coordinates.
• Figure 4: Same as Fig. \ref{['dipoleLoc']}, the two displayed cases correspond to lower source density hypotheses (see legend).
• Figure 5: Energy evolution of the dipole amplitude predicted for our simulation and compared with Auger data (shown in blue with their error bars). The red marker shows the mean value (calculated over the 300 different realizations) obtained for the simulations and the shaded area shows the range in which 90% of the simulations are found. The four panels correspond to some of the models displayed in Figs. \ref{['dipoleLoc']} and \ref{['dipoleLoc2']} (see legend and text).