Studies of Systematic Uncertainties in the HiRes-II Measurement of the   UHECR Spectrum
  We present studies of systematic uncertainties in the measurement of the ultra-high energy cosmic ray (UHECR) spectrum with the FADC detector of the High Resolution Fly's Eye experiment (HiRes-II). One source of uncertainties lies in the simulation of the energy dependent aperture of the air fluorescence detector. We study the impact of changes in the energy spectrum and composition that are used as input to the aperture simulation. We also compare aperture estimates for two different hadronic interaction models - QGSJet and SIBYLL. Systematic uncertainties may further be introduced by the modeling of the aerosol component of the atmosphere. We have repeated the HiRes-II monocular analysis using an atmospheric database with hourly entries instead of our measurement of the average aerosol content. We will discuss changes in reconstructed energies and in the resulting spectrum. 
Andreas Zech (for the HiRes Collaboration)