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Abstract: The Pierre Auger Observatory has already collected more ultra high energy cosmic ray data than all previous experiments. The hybrid detection technique provides results on the energy spectrum and arrival directions of the highest energy cosmic rays, and characterize the extensive air showers in order to probe characteristics of the primary particle and its interactions. The Pierre Auger Observatory is also sensitive to ultra high energy neutrinos and photons. In this paper recent results from the Pierre Auger Observatory including a limit on the diffuse flux of neutrinos above and limit on the fraction of photons to the total cosmic ray flux are shown. [Copyright &y& Elsevier]

Abstract: The identification of primary photons or specifying stringent limits on the photon flux is of major importance for understanding the origin of ultra-high energy (UHE) cosmic rays. UHE photons can initiate particle cascades in the geomagnetic field, which leads to significant changes in the subsequent atmospheric shower development. We present a Monte Carlo program allowing detailed studies of conversion and cascading of UHE photons in the geomagnetic field. The program named PRESHOWER can be used both as an independent tool or together with a shower simulation code. With the stand-alone version of the code it is possible to investigate various properties of the particle cascade induced by UHE photons interacting in the Earth''s magnetic field before entering the Earth''s atmosphere. Combining this program with an extensive air shower simulation code such as CORSIKA offers the possibility of investigating signatures of photon-initiated showers. In particular, features can be studied that help to discern such showers from the ones induced by hadrons. As an illustration, calculations for the conditions of the southern part of the Pierre Auger Observatory are presented. Program summary: Title of program:PRESHOWER 1.0 Catalogue identifier:ADWG Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADWG Program obtainable: CPC Program Library, Quen''s University of Belfast, N. Ireland Computer on which the program has been thoroughly tested:Intel-Pentium based PC Operating system:Linux, DEC-Unix Programming language used:C, FORTRAN 77 Memory required to execute with typical data:<100 kB No. of bits in a word:32 Has the code been vectorized?:no Number of lines in distributed program, including test data, etc.:2567 Number of bytes in distributed program, including test data, etc.:25 690 Distribution format:tar.gz Other procedures used in PRESHOWER:IGRF [N.A. Tsyganenko, National Space Science Data Center, NASA GSFC, Greenbelt, MD 20771, USA, http://nssdc.gsfc.nasa.gov/space/model/magnetos/data-based/geopack.html], bessik, ran2 [Numerical Recipes, http://www.nr.com]. Nature of the physical problem:Simulation of a cascade of particles initiated by UHE photon passing through the geomagnetic field above the Earth''s atmosphere. Method of solution: The primary photon is tracked until its conversion into pair or until it reaches the upper atmosphere. If conversion occurred each individual particle in the resultant preshower is checked for either bremsstrahlung radiation (electrons) or secondary gamma conversion (photons). The procedure ends at the top of atmosphere and the shower particle data are saved. Restrictions on the complexity of the problem: Gamma conversion into particles other than electron pair has not been taken into account. Typical running time: 100 preshower events with primary energy require a 800 MHz CPU time of about 50 min, with the simulation time for 100 events grows up to 500 min. [Copyright &y& Elsevier]

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.

The new, publicly available Monte Carlo program PRESHOWER is presented. It allows detailed studies on conversion and cascading of UHE photons in the geomagnetic field. When coupled to a standard air shower simulation program, PRESHOWER enables complete simulations of UHE photon-induced showers. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]