Enhancing the Pierre Auger Observatory to the – eV range: Capabilities of an Infill Surface Array

The Pierre Auger Observatory has been designed to study the highest-energy cosmic rays in nature ( E ⩾ 10 18.5 eV). The determination of their arrival direction, energy and composition is performed by the analysis of the atmospheric showers they produce. The Auger Surface Array will consist of 1600 water Cherenkov detectors placed in an equilateral triangular grid of 1.5 km spacing. The aim of this paper is to show that the addition of a “small” area of surface detectors at half or less the above mentioned spacing would allow a dramatic increase of the physical scope of this Observatory, reaching lower energies at which the transition from galactic to extragalactic sources is expected.