Depth of maximum of air-shower profiles at the Pierre Auger Observatory: Measurements above 1017.2 eV and composition implications

Abstract

We present distributions of shower depth of maximum (Xmax) and their interpretation in terms of the cosmic ray mass composition. The measurements of Xmax are based on data from the fluorescence telescopes of the Pierre Auger Observatory. Due to the extension of the field of view with the High Elevation Auger Telescopes, a lower energy threshold of 1017.2 eV can be reached. At the highest energies we have increased the available statistics by including three more years of data in the analysis compared to our previous publications. We present estimates of the first two moments of the Xmax distribution and of the composition fractions over a large energy range, from 1017.2 eV to about 1019.6 eV. The composition fractions are estimated by fitting the Xmax distributions with four elemental groups represented by p, He, N and Fe and using post-LHC hadronic models.