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


J. Abraham, Consejo Nacional de Investigaciones Científicas y Técnicas
P. Abreu, Instituto Superior Técnico
M. Aglietta, Università degli Studi di Torino
C. Aguirre, Universidad Catolica de Bolivia
E. J. Ahn, Fermi National Accelerator Laboratory
D. Allard, APC - AstroParticule et Cosmologie
I. Allekotte, Instituto Balseiro
J. Allen, New York University
P. Allison, The Ohio State University
J. Alvarez-Muñiz, Universidad de Santiago de Compostela
M. Ambrosio, Università degli Studi di Napoli Federico II
L. Anchordoqui, University of Wisconsin-Milwaukee
S. Andringa, Instituto Superior Técnico
A. Anzalone, INAF Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo
C. Aramo, Università degli Studi di Napoli Federico II
S. Argiró, Università degli Studi di Torino
K. Arisaka, University of California, Los Angeles
F. Arneodo, Laboratori Nazionali del Gran Sasso
F. Arqueros, Universidad Complutense de Madrid
T. Asch, Karlsruhe Institute of Technology, Campus North
H. Asorey, Instituto Balseiro
P. Assis, Instituto Superior Técnico
J. Aublin, Laboratoire de Physique Nucléaire et de Hautes Energies
M. Ave, The Enrico Fermi Institute
G. Avila, Pierre Auger Observatory
T. Bäcker, Universität Siegen
D. Badagnani, Universidad Nacional de La Plata
K. B. Barber, The University of Adelaide
A. F. Barbosa, Centro Brasileiro de Pesquisas Físicas
S. L.C. Barroso, Universidade Estadual do Sudoeste da Bahia
B. Baughman, The Ohio State University
P. Bauleo, Colorado State University

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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 ≡ 1018 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. © 2009 Elsevier B.V. All rights reserved.

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Astroparticle Physics