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

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.

proceedings of the 37th International Cosmic Ray Conference (ICRC 2021), 12-23 July 2021, Berlin, Germany

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• Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above $8\times 10^{18}$ eV
• Constraints on the origin of cosmic rays above $10^{18}$ eV from large scale anisotropy searches in data of the Pierre Auger Observatory

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• Constraining ultra-high-energy cosmic ray composition through cross-correlations
• Modelling cross-correlations of ultra-high-energy cosmic rays and galaxies
• Explaining the UHECR spectrum, composition and large-scale anisotropies with radio galaxies