Upper limit on the cosmic-ray photon fraction at EeV energies from the Pierre Auger Observatory Pierre Auger Collaboration Bueno Villar, Antonio García Gámez, Diego González, J. G. Lozano Bahílo, Julio Navarro, J. L. Navas Concha, Sergio Cosmic rays Ultra high energy (UHE) Photon From direct observations of the longitudinal development of ultra-high energy air showers performed with the Pierre Auger Observatory, upper limits of 3.8%, 2.4%, 3.5% and 11.7% (at 95% c.l.) are obtained on the fraction of cosmic-ray photons above 2, 3, 5 and 10 EeV (1 EeV = 10^18 eV) respectively. These are the first experimental limits on ultra-high energy photons at energies below 10 EeV. The results complement previous constraints on top-down models from array data and they reduce systematic uncertainties in the interpretation of shower data in terms of primary flux, nuclear composition and proton-air cross-section. 2013-11-08T10:45:37Z 2013-11-08T10:45:37Z 2009 info:eu-repo/semantics/article Pierre Auger Collaboration. Upper limit on the cosmic-ray photon fraction at EeV energies from the Pierre Auger Observatory. Astroparticle Physics, 31(6): 399-406 (2009). [http://hdl.handle.net/10481/29084] 0927-6505 arXiv:0903.1127v2 doi: 10.1016/j.astropartphys.2009.04.003 http://hdl.handle.net/10481/29084 eng http://creativecommons.org/licenses/by-nc-nd/3.0/ info:eu-repo/semantics/openAccess Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License Elsevier