Abstract
We report a measurement of the energy spectrum of cosmic rays for energies above $2.5 {\times} 10^{18}~$eV based on 215,030 events recorded with zenith angles below $60^\circ$. A key feature of the work is that the estimates of the energies are independent of assumptions about the unknown hadronic physics or of the primary mass composition. The measurement is the most precise made hitherto with the accumulated exposure being so large that the measurements of the flux are dominated by systematic uncertainties except at energies above $5 {\times} 10^{19}~$eV. The principal conclusions are: (1) The flattening of the spectrum near $5 {\times} 10^{18}~$eV, the so-called "ankle", is confirmed. (2) The steepening of the spectrum at around $5 {\times} 10^{19}~$eV is confirmed. (3) A new feature has been identified in the spectrum: in the region above the ankle the spectral index $\gamma$ of the particle flux ($\propto E^{-\gamma}$) changes from $2.51 \pm 0.03~{\rm (stat.)} \pm 0.05~{\rm (sys.)}$ to $3.05 \pm 0.05~{\rm (stat.)} \pm 0.10~{\rm (sys.)}$ before changing sharply to $5.1 \pm 0.3~{\rm (stat.)} \pm 0.1~{\rm (sys.)}$ above $5 {\times} 10^{19}~$eV. (4) No evidence for any dependence of the spectrum on declination has been found other than a mild excess from the Southern Hemisphere that is consistent with the anisotropy observed above $8 {\times} 10^{18}~$eV.
Highlights
The first cosmic rays having energies above 1019 eV were detected nearly 60 years ago [1,2], the question of their origin remains unanswered
The uncertainty in the reconstruction of the energy deposit ranges from 6.5% to 5.6% and accounts for the uncertainty associated with the modeling of the light spread away from the image axis and with the extrapolation of the modified Gaisser-Hillas profile beyond the field of view of the telescopes
This is a conservative estimate compared to that obtained from the fit, but this enables us to account for systematic changes that would have occurred had we chosen another functional shape for the parametrization
Summary
The first cosmic rays having energies above 1019 eV were detected nearly 60 years ago [1,2], the question of their origin remains unanswered. The Pierre Auger Collaboration introduced the concept of a hybrid observatory in which the bulk of the events used for spectrum determination is obtained with an array of detectors deployed on the ground and the integral of the longitudinal profile, measured using a fluorescence detector, is used to calibrate a shower-size estimate made with the ground array This hybrid approach has led to a substantial improvement in the accuracy of reconstruction of fluorescence events and to a calorimetric estimate of the energy of the primary particles for events recorded during periods when the fluorescence detector cannot be operated.
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