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

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 km^2 with 1500m spacing. Additionally, two nested arrays of the same kind with 750m and 433m spacing were utilized to lower the energy threshold of the measurements. This contribution presents, for the first time, the spectrum measured with the 433m array, which reduces the energy threshold down to 63 PeV, nearly half the energy at which we previously published a steepening using the 750m 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