Measurement of the cosmic-ray energy spectrum above2.5×1018  eVusing the Pierre Auger Observatory

Abstract

We report a measurement of the energy spectrum of cosmic rays for energies above 2.5 × 10 18 eV based on 215,030 events recorded with zenith angles below 60°. 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 × 10 19 eV. The principal conclusions are (1) The flattening of the spectrum near 5 × 10 18 eV, the so-called "ankle," is confirmed. (2) The steepening of the spectrum at around 5 × 10 19 eV is confirmed. (3) A new feature has been identified in the spectrum: in the region above the ankle the spectral index γ of the particle flux (∝ E −γ) changes from 2.51 AE 0.03 ðstatÞ AE 0.05 ðsystÞ to 3.05 AE 0.05 ðstatÞ AE 0.10 ðsystÞ before changing sharply to 5.1 AE 0.3 ðstatÞ AE 0.1 ðsystÞ above 5 × 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 × 10 18 eV.

Features of the Energy Spectrum of Cosmic Rays above 2.5×1018 eV Using the Pierre Auger Observatory

“…This spectral determination is unique in making no assumptions about the mass composition or the hadronic physics. Full details are reported in [8].…”

“…Accordingly, for each event, Sð1000Þ is adjusted to a reference value S 38 , the magnitude that it would have had, had the cosmic ray arrived at the median zenith angle of 38°. The long-established procedure for making this correction, the Constant Intensity Cut [11] method, relies on the quasi-isotropy of cosmic rays in zenith angle given the small anisotropy contrasts in celestial coordinates [8]. The large number of events has made it possible to refine the original approach and quantify the change in shower absorption as a function of energy.…”

“…Hybrid events are thus used to develop a calibration curve such that every estimate of S 38 can be assigned a valuation of E FD . Here 3338 hybrid events surviving rigorous quality cuts [8] are used to obtain a relationship between S 38 and E FD of the form E FD ¼ AS B 38 , where A ¼ð1.86AE0.03Þ×10 17 eV and B ¼ 1.031AE0.004. No zenithal dependence of A or B has been found, further validating the use of the constant intensity method [8].…”

“…Here 3338 hybrid events surviving rigorous quality cuts [8] are used to obtain a relationship between S 38 and E FD of the form E FD ¼ AS B 38 , where A ¼ð1.86AE0.03Þ×10 17 eV and B ¼ 1.031AE0.004. No zenithal dependence of A or B has been found, further validating the use of the constant intensity method [8]. Such a simple dependence is sufficient to describe the data in full detail.…”

“…The procedure for extracting the spectrum from the observations, fully discussed in [8], is summarized here.…”