B. Wilczynska

Upper limit on the photon fraction in highest-energy cosmic rays from AGASA data.

A new method to derive an upper limit on photon primaries from small data sets of air showers is developed which accounts for shower properties varying with the primary energy and arrival direction. Applying this method to the highest-energy showers recorded by the AGASA experiment, an upper limit on the photon fraction of 51% (67%) at a confidence level of 90% (95%) for primary energies above 1.25 * 10^20 eV is set. This new limit on the photon fraction above the GZK cutoff energy constrains the Z-burst model of the origin of highest-energy cosmic rays.

Optical image of an extensive air shower

Abstract Photons which constitute a shower image are not emitted simultaneously, but come from a range of shower front positions along the shower path. A spatial distribution of points of origin of photons arriving simultaneously to the eye is determined in this paper. Using realistic distribution of particles in a shower, and taking atmospheric scattering of light into account, the distribution of light arriving simultaneously to a detector is obtained. This instantaneous image (“a snapshot”) of the shower is independent of the detector properties – this is “what the shower looks like” when recorded with an ideal detector.

Simulation of air shower image in fluorescence light based on energy deposits derived from CORSIKA

Abstract Spatial distributions of energy deposited by an extensive air shower in the atmosphere through ionization, as obtained from the CORSIKA simulation program, are used to find the fluorescence light distribution in the optical image of the shower. The shower image derived in this way is somewhat smaller than that obtained from the NKG lateral distribution of particles in the shower. The size of the image shows a small dependence on the primary particle type.

On the primary particle type of the most energetic Fly's Eye event

The longitudinal profile of the 320 EeV event observed by the Fly’s Eye experiment is analysed. A method of testing the hypothesis of a specific primary particle type is described. Results for different particle types are summarized. For hadronic primaries between proton and iron nuclei, the discrepancy between observed and simulated profiles is in the range of 0.6-1.0σ for two different hadronic interaction models investigated. For primary photons, the discrepancy is 1.5σ assuming a standard extrapolation of the photonuclear cross-section with energy. Larger values of the cross-section at highest energies make primary photon showers more similar to hadron-initiated events. The influence of varying the extrapolation of the photonuclear cross-section is studied.

On a possible photon origin of the most-energetic AGASA events

In this work the ultra high energy cosmic ray events recorded by the AGASA experiment are analyzed. With detailed simulations of the extensive air showers initiated by photons, the probabilities are determined of the photonic origin of the 6 AGASA events for which the muon densities were measured and the reconstructed energies exceeded 1020 eV. On this basis a new, preliminary upper limit on the photon fraction in cosmic rays above 1020 eV is derived and compared to the predictions of exemplary top-down cosmic-ray origin models.