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

TL;DR

This study combines data from the Pierre Auger Observatory and Telescope Array to analyze large-scale anisotropy in ultra-high-energy cosmic rays above 10^{19} eV, finding no significant deviations from isotropy and setting upper limits on anisotropy amplitudes.

Contribution

First joint full-sky spherical harmonic analysis of cosmic rays above 10^{19} eV using two observatories, providing new constraints on anisotropy.

Findings

No significant anisotropy detected.

Upper limits on dipole amplitudes between 7% and 13%.

Upper limits on quadrupole amplitudes between 7% and 10%.

Abstract

Spherical harmonic moments are well-suited for capturing anisotropy at any scale in the flux of cosmic rays. An unambiguous measurement of the full set of spherical harmonic coefficients requires full-sky coverage. This can be achieved by combining data from observatories located in both the northern and southern hemispheres. To this end, a joint analysis using data recorded at the Telescope Array and the Pierre Auger Observatory above $10^{19}$ eV is presented in this work. The resulting multipolar expansion of the flux of cosmic rays allows us to perform a series of anisotropy searches, and in particular to report on the angular power spectrum of cosmic rays above $10^{19}$ eV. No significant deviation from isotropic expectations is found throughout the analyses performed. Upper limits on the amplitudes of the dipole and quadrupole moments are derived as a function of the direction in the sky, varying between 7% and 13% for the dipole and between 7% and 10% for a symmetric quadrupole.