Observation of the Shadows of the Moon and Sun Using the Pierre Auger Observatory at an Average Energy of $7\times10^{17}\,$eV

TL;DR

This paper reports the first observation of the Moon's shadow at ultra-high energies using the Pierre Auger Observatory, confirming directional accuracy and providing insights into cosmic ray interactions at energies around 7×10^{17} eV.

Contribution

First detection of the Moon's shadow at energies near 7×10^{17} eV, demonstrating the observatory's angular resolution and validating cosmic ray directional reconstruction.

Findings

Moon's shadow observed with >3σ significance

Effective angular resolution derived from shadow analysis

Sun's shadow also studied

Abstract

The interaction of cosmic rays with celestial bodies such as the Moon or the Sun produces a shadow in the arrival direction distribution of the cosmic rays reaching the Earth. Such deficits from an isotropic flux have been observed by astroparticle observatories below energies of $10^{15}\,$eV. Above this energy, measurements were limited due to the low number of events as a result of the steeply falling cosmic-ray flux with energy. With more than 10.6 million events recorded during 20 years of operation of the Pierre Auger Observatory, we report the first observation of the shadow of the Moon at an average energy of $7\times10^{17}\,$eV with a maximum significance above 3$\sigma$. The shadow is an end-to-end check that the celestial directions are correctly reconstructed from the air shower data, and it is used here to derive the effective angular resolution for this dataset. Additionally, we present the results of a similar study on the shadow of the Sun.