Upper limit on the cosmic-ray photon fraction at EeV energies from the Pierre Auger Observatory
astro-ph.HE
/ Authors
The Pierre Auger Collaboration, J. Abraham, P. Abreu, M. Aglietta, C. Aguirre, E. J. Ahn, D. Allard, I. Allekotte, J. Allen, P. Allison
and 451 more authors
J. Alvarez-Muñiz, M. Ambrosio, L. Anchordoqui, S. Andringa, A. Anzalone, C. Aramo, S. Argiró, K. Arisaka, F. Arneodo, F. Arqueros, T. Asch, H. Asorey, P. Assis, J. Aublin, M. Ave, G. Avila, T. Bäcker, D. Badagnani, K. B. Barber, A. F. Barbosa, S. L. C. Barroso, B. Baughman, P. Bauleo, J. J. Beatty
/ Abstract
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.