Upper limit on the cosmic-ray photon fraction at EeV energies from the Pierre Auger Observatory

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 equivalent to 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. (C) 2009 Elsevier B.V. All rights reserved. RI De Domenico, Manlio/D-1966-2009; Dias, Sandra/F-8134-2010; Dutan, Ioana/C-2337-2011; Caramete, Laurentiu/C-2328-2011; Aramo, Carla/D-4317-2011; Beatty, James/D-9310-2011; Pesce, Roberto/G-5791-2011; Kemp, Ernesto/H-1502-2011; Chiavassa, Andrea/A-7597-2012; Verzi, Valerio/B-1149-2012; Chinellato, Carola Dobrigkeit /F-2540-2011; Venters, Tonia/D-2936-2012; Pavlidou, Vasiliki/C-2944-2011; Fauth, Anderson/F-9570-2012; de souza, Vitor/D-1381-2012; Shellard, Ronald/G-4825-2012; Petrolini, Alessandro/H-3782-2011; Miele, Gennaro/F-3628-2010; fulgione, walter/I-5232-2012; D'Urso, Domenico/I-5325-2012; Bleve, Carla/J-2521-2012; martello, daniele/J-3131-2012; Valino, Ines/J-8324-2012; Brogueira, Pedro/K-3868-2012; Chinellato, Jose Augusto/I-7972-2012; Tamburro, Alessio/A-5703-2013; Falcke, Heino/H-5262-2012; Arneodo, Francesco/B-8076-2013; Anjos, Joao/C-8335-2013; Sarkar, Subir/G-5978-2011; Schussler, Fabian/G-5313-2013 OI Shellard, Ronald/0000-0002-2983-1815; D'Urso, Domenico/0000-0002-8215-4542; Brogueira, Pedro/0000-0001-6069-4073; Falcke, Heino/0000-0002-2526-6724; Arneodo, Francesco/0000-0002-1061-0510; Sarkar, Subir/0000-0002-3542-858X; Schussler, Fabian/0000-0003-1500-6571

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 equivalent to 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. (C) 2009 Elsevier B.V. All rights reserved.