Impact of the magnetic horizon on the interpretation of the Pierre Auger Observatory spectrum and composition data
Adila Abdul Halim, P. Abreu, M. Aglietta, I. Allekotte, Kévin Almeida Cheminant, A. Almela, Roberto Aloisio, Jaime Álvarez-Muñiz, Juan Ammerman Yebra, Gioacchino Alex Anastasi, Luis A. Anchordoqui, Belén Andrada, S. Andringa, L. Apollonio, C. Aramo, P.R Araújo Ferreira, Enrico Arnone, J.C. Arteaga Velázquez, P. Assis, G. Ávila, Emanuele Avocone, Alena Bakalová, Felicia Barbato, Adriel Bartz Mocellin, Jose A. Bellido, Corinne Bérat, M. E. Bertaina, Gopal Bhatta, Marta Bianciotto, Peter L. Biermann, Virginia Binet, Kathrin Bismark, Teresa Bister, J. Biteau, Jiří Blažek, C. Bleve, J. Blümer, M. Boháčová, Denise Boncioli, C. Bonifazi, L. Bonneau Arbeletche, Nataliia Borodai, J. Brack, P.G. Brichetto Orchera, F. L. Briechle, A. Bueno, S. Buitink, Mario Buscemi, Max Büsken, Anthony Bwembya, K. S. Caballero‐Mora, Sergio Cabana-Freire, Lorenzo Caccianiga, F. Campuzano, R. Caruso, A. Castellina, Fernando Catalani, G. Cataldi, Lorenzo Cazon, M. Cerda, A Cermenati, J.A. Chinellato, J. Chudoba, L. Chytka, R. W. Clay, A.C Cobos Cerutti, Roberta Colalillo, M. R. Coluccia, R. Conceição, Antonio Condorelli, Giovanni Consolati, M. Conte, Fabio Convenga, D. Correia dos Santos, Pedro J. Costa, C. E. Covault, M. Cristinziani, Carlo Salvattore Cruz Sanchez, S. Dasso, K. Daumiller, B. R. Dawson, R. M. de Almeida, J. de Jesús, S. J. de Jong, A. De Roeck, I. De Mitri, Jaime de Oliveira, Danelise de Oliveira Franco, F. de Palma, V. de Souza, Beatriz de Errico, Emanuele De Vito, A. Del Popolo, O. Deligny, N. Denner, L. Deval, Armando di Matteo, M. Dobre, C. Dobrigkeit, J. C. D'Olivo
Abstract
Abstract The flux of ultra-high energy cosmic rays reaching Earth above the ankle energy (5 EeV) can be described as a mixture of nuclei injected by extragalactic sources with very hard spectra and a low rigidity cutoff. Extragalactic magnetic fields existing between the Earth and the closest sources can affect the observed CR spectrum by reducing the flux of low-rigidity particles reaching Earth. We perform a combined fit of the spectrum and distributions of depth of shower maximum measured with the Pierre Auger Observatory including the effect of this magnetic horizon in the propagation of UHECRs in the intergalactic space. We find that, within a specific range of the various experimental and phenomenological systematics, the magnetic horizon effect can be relevant for turbulent magnetic field strengths in the local neighbourhood in which the closest sources lie of order B rms ≃ (50–100) nG (20 Mpc/d s )( 100 kpc/L coh ) 1/2 , with d s the typical intersource separation and L coh the magnetic field coherence length. When this is the case, the inferred slope of the source spectrum becomes softer and can be closer to the expectations of diffusive shock acceleration, i.e., ∝ E -2 . An additional cosmic-ray population with higher source density and softer spectra, presumably also extragalactic and dominating the cosmic-ray flux at EeV energies, is also required to reproduce the overall spectrum and composition results for all energies down to 0.6 EeV.