ar X iv : a st ro - p h / 04 11 06 0 v 1 2 N ov 2 00 4 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 analysed. 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 10 20 eV. On this basis a new, preliminary upper limit on the photon fraction in cosmic rays above 10 20 eV is derived and compared to the predictions of exemplary top-down cosmic-ray origin models. Studies on ultra-high energy photons as primary cosmic rays have the potential to discern between different models of origin of ultra-high energy cosmic rays (UHECR). Many top-down scenarios predict a dominance of photons in the CR flux above 10 20 eV (for a review see [1]), while according to classical acceleration scenarios in this energy range the CR flux should consist mainly of hadrons. Thus, any experimental conclusions about a fraction of photons in UHECR, e.g. an upper limit on this fraction, are expected to provide constraints for the models of UHECR origin. Presently, there are few experimental upper limits on photon fraction estimated on the basis of cosmic rays of energies above 10 19 eV ([2], [3]). They are plotted in Fig. 1 and compared to the predictions of the three exemplary top-down models: super heavy dark matter (SHDM) [4], Z-bursts (ZB) and topological defects (TD) [5]. It can be seen that the available limits are not very constraining since below 5×10 19 eV a typical predicted photon fraction is still low. Above 10 20 eV, where the photon fraction predictions are larger, there is no experimental upper limit, although there are a few well documented cosmic ray events of such extremely large energies. While large amount of UHECR data from next generation giant cosmic-ray detectors is still on their way, in this work the available data of extensive air showers detected by the AGASA experiment are analysed to derive an upper limit on the photon fraction in UHECR. In order to determine the probability of an event being initiated by a photon the muon densities measured on ground, which are expected to be good indicators of primary mass [6,7], were compared to the simulations. 2. AGASA cosmic ray data above 10 20 eV Among the 11 cosmic ray events above 10 20 eV recorded by the AGASA experiment [8,2] in …