Acceptance of fluorescence detectors and its implication in energy spectrum inference at the highest energies

In past years, HiRes and AGASA experiments have explored the fluorescence and the ground array experimental techniques to measure extensive air showers, both being essential to investigate the ultrahigh energy cosmic rays. However, such collaborations have published contradictory energy spectra for energies above the Greisen-Zatsepin-Kuzmin cutoff. In this article, we investigate the acceptance of fluorescence telescopes to different primary particles at the highest energies. Using CORSIKA and CONEX shower simulations without and with the new preshowering scheme, which allows photons to interact in the Earth's magnetic field, we estimate the aperture of the HiRes-I telescope for gamma, iron nucleus, and proton primaries as a function of the number of simulated events and primary energy. We also investigate the possibility that systematic differences in shower development for hadrons and gammas could mask or distort vital features of the cosmic ray energy spectrum at energies above the photo-pion production threshold. The impact of these effects on the true acceptance of a fluorescence detector is analyzed in the context of top-down production models.