The paper is based on the long-period data acquired by the Pierre Auger Observatory. The presented results concern the analysis of the unique data set encompassing 4000 atmospheric air showers detected simultaneously by the surface and fluorescence detectors, while the observed parameter was the depth of shower maximum estimated from the fluorescence detector measurement. A transition from light to heavy elements with increasing energy is observed in the spectrum of incoming primary cosmic rays.
The fluorescence detector is one of two measurement systems of the hybrid design of the Observatory. Our group contributed to construction of the telescopes and we are responsible for their operation and monitoring. J. Ridky served as the fluorescence detector task leader during the time when majority of the presented data was collected. Concerning the physics analysis in this paper our team members were engaged in various data cross-checks.
⟨Xmax⟩ as a function of energy. Lines denote a fit with a broken line in lgE. The systematic uncertainties of ⟨Xmax⟩ are indicated by a dashed line. The number of events in each energy bin is displayed below the data points. HiRes data  are shown for comparison.