Combined fit of the energy spectrum and mass composition across the ankle with the data measured at the Pierre Auger Observatory
Eleonora Guido, P. Abreu, M. Aglietta, Justin M. Albury, I. Allekotte, Alejandro Almela, Jaime Álvarez-Muñiz, Rafael Alves Batista, Gioacchino Alex Anastasi, Luis A. Anchordoqui, Belén Andrada, S. Andringa, C. Aramo, Paulo Ricardo Araújo Ferreira, Juan Carlos Arteaga Velázquez, Hernán Gonzalo Asorey, P. Assis, Gualberto Avila, Alina Mihaela Badescu, Alena Bakalová, Alexandru Balaceanu, Felicia Barbato, Ricardo Jorge Barreira Luz, Karl‐Heinz Becker, Jose A. Bellido, Corinne Bérat, Mario E. Bertaina, X. Bertou, Peter L. Biermann, Virginia Binet, Kathrin Bismark, Teresa Bister, Jonathan Biteau, Jiří Blažek, C. Bleve, Martina Boháčová, Denise Boncioli, C. Bonifazi, Luan Bonneau Arbeletche, Nataliia Borodai, Ana Martina Botti, Jeffrey Brack, T. Bretz, P. Gabriel Brichetto Orchera, Florian Lukas Briechle, P. Buchholz, A. Bueno, S. Buitink, Mario Buscemi, Max Büsken, K. S. Caballero‐Mora, Lorenzo Caccianiga, Fabrizia Canfora, Ioana Caracas, Juan Miguel Carceller, R. Caruso, A. Castellina, Fernando Catalani, G. Cataldi, Lorenzo Cazon, Marcos Cerda, J. A. Chinellato, J. Chudoba, L. Chytka, R. W. Clay, Agustín Cobos Cerutti, Roberta Colalillo, Alan Coleman, Maria Rita Coluccia, R. Conceição, Antonio Condorelli, Giovanni Consolati, F. Contreras, Fabio Convenga, Diego Correia dos Santos, C. E. Covault, S. Dasso, K. Daumiller, B. R. Dawson, Jarryd A. Day, Rogerio M. de Almeida, Joaquin de Jesus, S. J. de Jong, G. De Mauro, João de Mello Neto, I. De Mitri, Jaime de Oliveira, Danelise de Oliveira Franco, Francesco de Palma, Vitor de Souza, Emanuele De Vito, Mariano del Río, Olivier Deligny, Luca Deval, Armando di Matteo, C. Dobrigkeit, Juan Carlos D’Olivo, Luis Miguel Domingues Mendes, Rita Cassia dos Anjos, Diego dos Santos
Abstract
The combined fit of the energy spectrum and mass composition data above 5 EeV suggested the presence of extragalactic sources ejecting ultra-high-energy cosmic rays with relatively low maximum energies, hard spectral indices and mixed chemical compositions, dominated by the contribution of intermediate mass groups. Here we present an extension of the fit to lower energies, to include the feature observed near 5 EeV in the all-particle energy spectrum, the so-called ankle. We show that it is possible to generate such a change of slope assuming that the flux below the ankle is provided by the superposition of different contributions. The simplest extension of this sort consists of introducing a supplemental extragalactic component at low energy, characterised by different physical parameters with respect to the one being dominant above the ankle: such a component may originate from a different population of sources or be provided by interactions occurring in the acceleration sites. In this framework we also explore the possibility of including the end of a Galactic contribution at low energies. The fit suggests that these scenarios provide a reasonable description of the measurements across the ankle, without significantly affecting the interpretation obtained for the above-ankle region. In order to evaluate our capability to constrain the source models, we finally discuss the impact of the main experimental systematic uncertainties and of the theoretical models choice on the fit results.