Key data properties:
| item | value |
|---|---|
| technique | air-fluorescence, monocular mode (both stations, Dugway, Utah) |
| energy resolution | ~17% (HiRes-I, profile-constrained fit), ~12% (HiRes-II) at high energy |
| triple-power-law fit | χ²/DOF = 35.1/35 (vs 63.0/37 for single-break model) |
| systematic uncertainties | energy scale 17%, flux 30% (PMT calibration 10%, fluorescence yield 6%, missing energy 5%, aerosols 5%, mean dE/dx 10%, in quadrature) |
| energy range covered | 10^17.2 to above 10^20.5 eV |
The measured spectrum is in direct tension with AGASA’s earlier ground-array result of an unbroken spectrum beyond 10^20 eV.
Methodology
Nitrogen UV fluorescence from extensive air showers imaged by two stations (22 and 42 mirror-PMT telescope modules); photometric calibration via monthly Xenon flash lamps + nightly YAG laser (~10% RMS photometric accuracy); atmospheric attenuation corrected using measured vertical aerosol optical depth (mean 0.04 ± 0.02); shower profiles fit to the Gaisser-Hillas function giving near-calorimetric energy; ~10% correction for unobserved neutrino/muon energy (CORSIKA/QGSJet, Sibyll within 2%); aperture from full Monte Carlo validated by data-MC comparisons of shower distance and brightness distributions. Systematics are largely independent of AGASA’s ground-array/lateral-distribution method.