Progress towards stereo observation of ultra-high-energy cosmic rays with Fluorescence detector Array of Single-pixel Telescopes
Shunsuke Sakurai, Justin Albury, Jose Bellido, Fraser Bradfield, Karel Cerny, Ladislav Chytka, John Farmer, Toshihiro Fujii, Petr Hamal, Pavel Horvath, Miroslav Hrabovsky, Vlastimil Jilek, Jakub Kmec, Jiri Kvita, Max Malacari, Dusan Mandat, Massimo Mastrodicasa, John N. Matthews, Stanislav Michal, Hiromu Nagasawa, Hiroki Namba, Marcus Niechciol, Libor Nozka, Miroslav Palatka, Miroslav Pech, Paolo Privitera, Francesco Salamida, Petr Schovanek, Radomir Smida, Daniel Stanik, Zuzana Svozilikova, Akimichi Taketa, Kenta Terauchi, Stan B. Thomas, Petr Travnicek, Martin Vacula
TL;DR
This article assesses FAST, a cost-efficient fluorescence detector comprised of single-pixel telescopes designed to observe extensive air showers from UHECRs with $E>10^{19}$ eV. It situates FAST within the UHECR science case, emphasizing the role of $X_{max}$ for composition and the need for large-area coverage. The work reports prototype deployments at Auger and TA, with thousands of live hours and initial energy and $X_{max}$ measurements, alongside cross-calibration opportunities such as EarthCARE UV laser events. It outlines a stereo-observation program, including new FAST designs (FAST-Field) and additional units at Auger to form a small and eventually triangular array, aiming for about 100 stereo events per year and $X_{max}$ precision of about 15 g cm$^{-2}$ and energy resolution of about 10%, supported by machine-learning initializations for top-down reconstruction. Overall, FAST offers a scalable, low-cost path to high-statistics UHECR data and cross-hemisphere calibration.
Abstract
Ultra-high-energy cosmic rays (UHECRs) are the most energetic particles ever detected. Cosmic rays that achieve the highest energies are rare, and their flux at Earth is extremely low. As a result, next-generation experiments with large effective areas are required and under development. The Fluorescence detector Array of Single-pixel Telescopes (FAST) is one such project. Although observation time is limited compared with ground particle detectors, it enables direct measurements of $X_\mathrm{max}$, a crucial parameter sensitive to the primary cosmic-ray composition. FAST will achieve large-area coverage by significantly reducing the cost of telescopes. This necessitates a simplified telescope compared to conventional designs. Demonstrating the feasibility of our telescope and observational method is essential. To validate the FAST concept, prototype telescopes have been deployed at the Pierre Auger Observatory and the Telescope Array experiment.