Update on the searches for anisotropies in UHECR arrival directions with the Pierre Auger Observatory and the Telescope Array
Lorenzo Caccianiga, Adila Abdul Halim, P. Abreu, M. Aglietta, I. Allekotte, Kévin Almeida Cheminant, Alejandro Almela, Roberto Aloisio, Jaime Álvarez-Muñiz, Juan Ammerman Yebra, Gioacchino Alex Anastasi, Luis A. Anchordoqui, Belén Andrada, S. Andringa, C. Aramo, Paulo Ricardo Araújo Ferreira, Enrico Arnone, Juan Carlos Arteaga Velázquez, Hernán Gonzalo Asorey, P. Assis, Gualberto Avila, Emanuele Avocone, Alina Mihaela Badescu, Alena Bakalová, Alexandru Balaceanu, Felicia Barbato, Adriel Bartz Mocellin, Jose A. Bellido, Corinne Bérat, Mario E. Bertaina, Gopal Bhatta, Marta Bianciotto, Peter L. Biermann, Virginia Binet, Kathrin Bismark, Teresa Bister, Jonathan Biteau, Jiří Blažek, C. Bleve, Johannes Blümer, Martina Boháčová, Denise Boncioli, C. Bonifazi, Luan Bonneau Arbeletche, Nataliia Borodai, J. Brack, P. Gabriel Brichetto Orchera, Florian Lukas Briechle, A. Bueno, S. Buitink, Mario Buscemi, Max Büsken, Anthony Bwembya, K. S. Caballero‐Mora, Sergio Cabana-Freire, Ioana Caracas, R. Caruso, A. Castellina, Fernando Catalani, G. Cataldi, Lorenzo Cazon, Marcos Cerda, Alessandro Cermenati, 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, Matteo Conte, Fabio Convenga, Diego Correia dos Santos, Pedro J. Costa, C. E. Covault, M. Cristinziani, Carlo Salvattore Cruz Sanchez, S. Dasso, K. Daumiller, B. R. Dawson, Rogerio M. de Almeida, Joaquin de Jesus, S. J. de Jong, João de Mello Neto, I. De Mitri, Jaime de Oliveira, Danelise de Oliveira Franco, Francesco de Palma, Vitor de Souza, Emanuele De Vito, Antonino Del Popolo, Olivier Deligny, Nikolas Denner, Luca Deval, Armando di Matteo, Madalina Dobre
Abstract
The origin of ultra-high-energy cosmic rays (UHECRs), particles from outer space with energies~$E \geq 1\;$EeV, is still unknown, though the near-isotropy of their arrival direction distribution excludes a dominant Galactic contribution, and interactions with background photons prevent them from travelling cosmologically large distances. This suggests that their sources must be searched for in nearby galaxy groups and clusters. Deflections by intergalactic and Galactic magnetic fields are expected to hinder such searches but not preclude them altogether. So far, the only anisotropy detected with statistical significance $\geq 5\sigma$ is a modulation in right ascension in the data from the Pierre Auger Observatory at~$E \geq 8\;$EeV interpretable as a 7% dipole moment. Various hints for higher-energy, smaller-scale anisotropies have been reported. UHECR arrival direction data from both the Pierre Auger Observatory and the Telescope Array experiment have been searched for anisotropies by a working group with members from both collaborations; combining the two datasets requires a cross-calibration procedure due to the different systematic uncertainties on energy measurements but allows us to perform analyses that are less model-dependent than what can be done with partial sky coverage. We report a significant dipole pointing away from the Galactic Center and a ${\sim}4.6\sigma$ anisotropy found when comparing the directions of UHECRs with a catalog of starburst galaxies.