Auger@TA: In-situ Cross-Calibration of the World's Largest Cosmic Ray Observatories

TL;DR

Addressing the persistent discrepancy between the Auger and TA UHECR energy spectra, the paper proposes an in-situ cross-calibration approach using a micro-array embedded in TA. The Auger@TA design deploys a hexagonal array of eight SD stations (seven Auger-like plus a center triplet with a single PMT) equipped with Water Cherenkov Detectors (WCD) and Surface Scintillator Detectors (SSD) to enable event-by-event cross-checks of the same showers, coordinated by a tailored Central Data Acquisition System (CDAS) and a robust radio/remote-access network. Early commissioning demonstrates a reliable communications chain, a simplified DAQ, and initial T2/T3 coincidences across four stations, with full hexagon expected by year-end. The anticipated outcome is a tighter cross-hemisphere energy-scale alignment, enabling joint Auger–TA spectral analyses and a clearer discrimination between astrophysical and detector-systematic contributions to the flux differences, particularly for $E \lesssim 10^{19.5}$ eV.

Abstract

The Pierre Auger Observatory (Auger) and the Telescope Array (TA) are the world's two largest ultra-high-energy cosmic ray (UHECR) observatories. They operate in the Southern and Northern hemispheres, respectively, at similar latitudes but with distinct surface detector (SD) designs. A significant challenge in studying UHECR physics across the full sky is the apparent discrepancy in flux measurements between the two experiments. This discrepancy could arise from astrophysical differences and/or systematic effects related to their detector designs and sensitivities to extensive air shower components. To address this, the Auger@TA working group aims to cross-calibrate the two observatories with a self-triggering micro-Auger array within the TA array. This micro-array consists of eight Auger Surface Detector (SD) stations equipped with Water Cherenkov Detectors (WCDs) and AugerPrime Surface Scintillator Detectors. Seven SD stations, configured with a centered-1-PMT design, are arranged in a hexagonal pattern with one station in the center, with 1.5 km spacing, mirroring the Auger layout. The eighth station, which features a standard 3-PMT Auger station, is located in conjunction with a TA detector at the center of the hexagon, forming a triplet for high-statistics and low-uncertainty cross-calibration. A custom communication system that uses readily available components enables seamless communication between stations and remote access to each station through a central computer. The micro-array is now fully deployed, and initial data-taking is about to start. This presentation will detail the instrumentation, communication systems, central data acquisition system, expected performance of the micro-array, and preliminary results as appropriate.

Figures (8)

• Figure 1: Comparison of the ultra-high energy cosmic-ray spectra measured by TA and Auger. Black points: TA standard analysis (14 years of SD data, TA fluorescence yield and missing‐energy corrections). Red points: TA with AirFly fluorescence yield and Auger missing‐energy correction (standard MC method). Blue triangles: TA with AirFly and Auger corrections using the CIC method. Orange open circles: Auger 2020 results kim_23.
• Figure 2: (A) Micro-array location in relation of TA's Black Rock Mesa FD, (B) The Auger@TA central stations, (C) Location of micro-array within TA's site.
• Figure 3: Auger@TA Station.
• Figure 4: The left side shows the hexagonal micro-array with its nine stations (eight Auger stations and one TA station). Each station communicates with the central computer through a radio antenna installed on a mast on each station. The right side shows CDAS installed and running on Frodo, the central computer and each of the processes and their schematics.
• Figure 5: An obtained muon charge histogram for station Sam.