UHECR arrival directions in the latest data from the original Auger and TA surface detectors and nearby galaxies

TL;DR

This study analyzes ultra-high-energy cosmic-ray arrival directions using combined data from Auger and TA observatories, finding significant correlations with nearby starburst galaxies at the highest energies, suggesting potential source associations.

Contribution

It introduces a combined analysis of UHECR data from Auger and TA, testing models with candidate sources and magnetic deflections, revealing a significant correlation with nearby starburst galaxies.

Findings

Significant correlation (4.2σ) between UHECRs and nearby starburst galaxies.

Detection of anisotropy at angular scales around 15.5° for energies above 38-49 EeV.

Weaker correlation observed with the overall galaxy distribution.

Abstract

The distribution of ultra-high-energy cosmic-ray arrival directions appears to be nearly isotropic except for a dipole moment of order $6 \times (E/10~\mathrm{EeV})$ per cent. Nonetheless, at the highest energies, as the number of possible candidate sources within the propagation horizon and the magnetic deflections both shrink, smaller-scale anisotropies might be expected to emerge. On the other hand, the flux suppression reduces the statistics available for searching for such anisotropies. In this work, we consider two different lists of candidate sources: a sample of nearby starburst galaxies and the 2MRS catalog tracing stellar mass within $250~\mathrm{Mpc}$. We combine surface-detector data collected at the Pierre Auger Observatory until 2020 and the Telescope Array until 2019, and use them to test models in which UHECRs comprise an isotropic background and a foreground originating from the candidate sources and randomly deflected by magnetic fields. The free parameters of these models are the energy threshold, the signal fraction, and the search angular scale. We find a correlation between the arrival directions of $11.8\%_{-3.1\%}^{+5.0\%}$ of cosmic rays detected with $E \ge 38~\mathrm{EeV}$ by Auger or with $E \gtrsim 49~\mathrm{EeV}$ by TA and the position of nearby starburst galaxies on a ${15.5^\circ}_{-3.2^\circ}^{+5.3^\circ}$ angular scale, with a $4.2\sigma$ post-trial significance, as well as a weaker correlation with the overall galaxy distribution.