Search for First Harmonic Modulation in the Right Ascension Distribution of Cosmic Rays Detected at the Pierre Auger Observatory

TL;DR

This study searches for dipolar anisotropies in cosmic ray arrival directions above 2.5×10^{17} eV using Pierre Auger Observatory data, setting stringent upper limits on first harmonic modulation amplitudes.

Contribution

It provides the most stringent upper limits on dipolar anisotropies in cosmic rays at EeV energies and compares these results with previous experiments and theoretical models.

Findings

Upper limits on anisotropy amplitudes are below 2% at 99% confidence level.

No significant dipolar anisotropy detected in the analyzed energy range.

Results constrain models of cosmic ray origin and propagation.

Abstract

We present the results of searches for dipolar-type anisotropies in different energy ranges above $2.5\times 10^{17}$ eV with the surface detector array of the Pierre Auger Observatory, reporting on both the phase and the amplitude measurements of the first harmonic modulation in the right-ascension distribution. Upper limits on the amplitudes are obtained, which provide the most stringent bounds at present, being below 2% at 99% $C.L.$ for EeV energies. We also compare our results to those of previous experiments as well as with some theoretical expectations.