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The second knee in the cosmic ray spectrum observed with the surface detector of the Pierre Auger Observatory

par Abdul Halim, A.;Zapparrata, Orazio ;Maris, Ioana Codrina ;Schlüter, Felix ; [et al.]
Référence (26 July 2023 through 3 August 2023: Nagoya), 38th International Cosmic Ray Conference, ICRC 2023, Pos proceedings of science (444), 398
Publication Publié, 2024-02-01

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Résumé :

The determination of the energy spectrum features with low systematic uncertainty is crucial for interpreting the nature of cosmic rays. In this study, we conducted a measurement of the energy spectrum at the Pierre Auger Observatory using a surface detector with a calorimetric energy scale indirectly set by a fluorescence detector. The surface detector consists of an array of water-Cherenkov detectors that extends over 3000 km2 with 1500 m spacing. Additionally, two nested arrays of the same kind with 750 m and 433 m spacing were utilized to lower the energy threshold of the measurements. This contribution presents, for the first time, the spectrum measured with the 433 m array, which reduces the energy threshold down to 63 PeV, nearly half the energy at which we previously published a steepening using the 750 m array. Our measurements include a characterization of the spectral features of the flux steepening around 230 PeV, known as the second-knee. The study benefits from a nearly 100% duty cycle and geometrical exposure. Notably, this is the first simultaneous measurement of the second knee energy and spectral indexes before and after the break, using a surface detector with an energy scale predominantly independent of air shower simulations and assumptions regarding hadronic interaction models.