Sensitivity of full-sky experiments to large scale cosmic ray anisotropies

TL;DR

This paper evaluates the potential of space-based full-sky experiments like EUSO to detect large-scale anisotropies in ultra-high-energy cosmic rays, highlighting their advantages over ground-based partial-sky observatories.

Contribution

It introduces a method to compare the sensitivity of full-sky space experiments with ground-based observatories for detecting cosmic ray anisotropies, using simulations of simplified sky models.

Findings

Space-based experiments have increased sensitivity to large-scale anisotropies.

Full-sky coverage improves the ability to identify multipole moments.

Simulations show significant advantages of space-based over ground-based observations.

Abstract

The two main advantages of space-based observation of extreme energy ($\gtrsim5\times10^{19}$ eV) cosmic rays (EECRs) over ground based observatories are the increased field of view and the full-sky coverage with nearly uniform systematics across the entire sky. The former guarantees increased statistics, whereas the latter enables a clean partitioning of the sky into spherical harmonics. The discovery of anisotropies would help to identify the long sought origin of EECRs. We begin an investigation of the reach of a full-sky space-based experiment such as EUSO to detect anisotropies in the extreme-energy cosmic-ray sky compared to ground based partial-sky experiments such as the Pierre Auger Observatory and Telescope Array. The technique is explained here, and simulations for a Universe with just two nonzero multipoles, monopole plus either dipole or quadrupole, are presented. These simulations quantify the advantages of space-based, all-sky coverage.