J. Abraham, Universidad Tecnologica Nacional
P. Abreu, Instituto Superior Técnico
M. Aglietta, Università degli Studi di Torino
C. Aguirre, Universidad Catolica de Bolivia
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, Istituto Nazionale di Fisica Nucleare, Sezione di Napoli
L. Anchordoqui, Northeastern University
S. Andringa, Instituto Superior Técnico
A. Anzalone, INAF Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo
C. Aramo, Istituto Nazionale di Fisica Nucleare, Sezione di Napoli
S. Argirò, Università degli Studi di Torino
K. Arisaka, University of California, Los Angeles
E. Armengaud, APC - AstroParticule et Cosmologie
F. Arneodo, Laboratori Nazionali del Gran Sasso
F. Arqueros, Universidad Complutense de Madrid
T. Asch, Karlsruher Institut für Technologie, Campus Nord
H. Asorey, Centro Atomico Bariloche
P. Assis, Instituto Superior Técnico
B. S. Atulugama, Pennsylvania State University
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
A. F. Barbosa, Centro Brasileiro de Pesquisas Físicas
D. Barnhill, University of California, Los Angeles
S. L.C. Barroso, Universidade Estadual do Sudoeste da Bahia
P. Bauleo, Colorado State University
J. J. Beatty, The Ohio State University

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A method is developed to search for air showers initiated by photons using data recorded by the surface detector of the Auger Observatory. The approach is based on observables sensitive to the longitudinal shower development, the signal risetime and the curvature of the shower front. Applying this method to the data, upper limits on the flux of photons of 3.8 × 10- 3, 2.5 × 10- 3, and 2.2 × 10- 3 km- 2 sr- 1 yr- 1 above 1019 eV, 2 × 1019 eV, and 4 × 1019 eV are derived, with corresponding limits on the fraction of photons being 2.0%, 5.1%, and 31% (all limits at 95% c.l.). These photon limits disfavor certain exotic models of sources of cosmic rays. The results also show that the approach adopted by the Auger Observatory to calibrate the shower energy is not strongly biased by a contamination from photons. © 2008 Elsevier B.V. All rights reserved.

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Astroparticle Physics

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