Measurement of the cosmic-ray energy spectrum above 2.5 EeV using 19 years of operation of the Pierre Auger Observatory

Abstract: We present the spectrum of cosmic rays with energies above 2.5 EeV measured at the Pierre Auger Observatory after 19 years of operation, covering the period before the AugerPrime upgrade. Two independent event sets from the surface array of 1500 m-spaced detectors are combined, yielding a total exposure of approximately 100,000 km$^2$ sr yr. The first set includes events with zenith angles less than 60$^\circ$, while the second consists of events between 60$^\circ$ and 80$^\circ$, for which azimuthal asymmetries must be accounted for in the energy estimator. The threshold energy is chosen to ensure a trigger efficiency of the surface detector greater than 97%, thus minimizing composition biases. The energy scale is determined using high-quality fluorescence measurements, providing calorimetric estimates without reliance on simulations. A statistically successful combination is achieved within the uncorrelated systematic uncertainties of the individual spectra. All spectra are consistent when analyzing potential declination dependences, except for a mild modulation expected from the previously reported dipolar anisotropy. In particular, this statement applies to the northernmost declination band [+25$^\circ$,+45$^\circ$], where only contribute events with zenith angles between 60$^\circ$ and 80$^\circ$. Beyond the firmly established ankle and suppression spectral features, the combined spectrum across declinations $-90^\circ$ to +45$^\circ$ provides high-precision measurements of the instep feature with more than 5$\sigma$ confidence.