The surface detector array of the Pierre Auger Observatory measures the arrival time distribution of particles, providing indirect information on the longitudinal development of the muonic component of extensive air showers. In this work, the depth at which the muon production is maximum and the corresponding fluctuations are reconstructed for more than 2000 events above 1.5 × 10 19 eV, in a wide range of zenith angles, between 45 • and 65 • . Both observables are exploited to gain insight

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... the most up-to-date hadronic interactions models, which are used in the simulation of extensive air showers. By constraining the models, these observables could allow one to obtain better information on high energy hadronic interactions, and therefore they could indirectly help for the mass composition study of ultra-high energy cosmic rays, one of the most intriguing issues of modern astrophysics.