Depth of maximum of air-shower profiles at the Pierre Auger Observatory. I. Measurements at energies above1017.8  eV

Abstract

We report a study of the distributions of the depth of maximum, Xmax, of extensive air-shower profiles with energies above 10 17.8 eV as observed with the fluorescence telescopes of the Pierre Auger Observatory. The analysis method for selecting a data sample with minimal sampling bias is described in detail as well as the experimental cross-checks and systematic uncertainties. Furthermore, we discuss the detector acceptance and the resolution of the Xmax measurement and provide parameterizations thereof as a function of energy. The energy dependence of the mean and standard 4 deviation of the Xmax-distributions are compared to air-shower simulations for different nuclear primaries and interpreted in terms of the mean and variance of the logarithmic mass distribution at the top of the atmosphere.

Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory

“…The decrease at the highest energies, observed at a high degree of significance, can be ascribed to interactions with background radiation [7,8] and/or to a maximum energy of cosmic rays at the sources. Concerning the mass composition, the average X max measured at the Pierre Auger Observatory (as discussed in [5,9,10]) indicates that UHECRs become heavier with increasing energy above 2 · 10 18 eV. Moreover, the measured fluctuations of X max indicate a small massdispersion at energies above the ankle.…”

Composition at the ``ankle'' measured by the Pierre Auger Observatory: pure or mixed?

“…The analysis is based on the same hybrid events as used in [7] recorded by both fluorescence (FD) and surface detectors (SD) of the Pierre Auger Observatory during the period from 01.01.2004 to 31.12.2012. The procedure of data selection, described in [7], guarantees that only high quality events are included in the analysis and that the mass composition of the selected sample is unbiased.…”

“…Thus r G is insensitive to the systematic effects on X * max or S * (1000) that might lead to shift or multiplication of these observables by a constant value. In particular we have checked that the recent changes in Auger energy and X max scales [19,7] do not change the observed correlation. The same insensitivity of r G was observed with respect to the application of various FD selection cuts which have been used in our publications from 2010 [20] until 2014 [7].…”

“…In particular we have checked that the recent changes in Auger energy and X max scales [19,7] do not change the observed correlation. The same insensitivity of r G was observed with respect to the application of various FD selection cuts which have been used in our publications from 2010 [20] until 2014 [7]. Finally, we introduced arbitrary energy and zenith angle dependent biases in X * max (up to 10 g cm −2 ) and S * (1000) (up to 10%) and this changed r G by 0.01.…”

“…The procedure of data selection, described in [7], guarantees that only high quality events are included in the analysis and that the mass composition of the selected sample is unbiased. The use of the signal in ground stations requires an additional application of the fiducial trigger cut [8] (the station with the highest signal should have at least 5 working neighbour stations), and exclusion of events with stations having saturated signal traces.…”