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The Pierre Auger Observatory (Auger) and the Telescope Array Project (TA) are the two largest ultra-high-energy cosmic ray observatories in the world. They operate in the Southern and Northern hemispheres, respectively, at similar latitudes but with different surface detector (SD) designs. This difference in detector design changes their sensitivity to the various components of extensive air showers. The over-arching goal of the Auger@TA working group is to cross-calibrate the SD arrays of the two observatories in order to identify or rule out systematic causes for the apparent differences in the flux measured at Auger and TA. The project itself is divided into two phases. Phase-I finished in 2020 and consisted of a station-level comparison facilitated by the deployment of two Auger stations, one prototype station with a single central PMT and a standard Auger station, in the middle of the TA SD near the Central Laser Facility, along with a modified TA station to provide external triggers from the TA SD. This provided the opportunity to observe the same extensive air showers with both Auger and TA detectors to directly compare their measurements. Phase-II of Auger@TA is currently underway and aims at building a self-triggering micro-Auger-array inside the TA array. This micro-array consists of eight Auger stations, seven of which use a 1-PMT prototype configuration and form a single hexagon with a traditional 1.5 km Auger spacing. The 8th station is of the standard Auger 3-PMT configuration and is placed at the center of the hexagon, along with a TA station to form a triplet. Each Auger station will also be outfitted with an AugerPrime Surface Scintillator Detector. A custom communication system using readily available components will be used to provide communication between the stations and remote access to each station via a central communications station. The deployment of the micro-array took place at the end of September 2022. A simulation study was carried out to gauge the expected performance of the Auger@TA micro-array and to derive trigger effi ciencies and event rates.

We report on the first results of a unique in-situ experimental cross-calibration effort of the surface detector of the Pierre Auger Observatory and of the Telescope Array experiment (Auger@TA). In the first phase of Auger@TA, we performed surface detector station-to-station comparisons for a collection of extensive air showers landing near the experimental setup and detected by Telescope Array. Beyond the deduced cross-calibration curve between the Water-Cherenkov-based Auger and Scintillator-based TA Surface Detector stations, we also investigate the consistency of their response for individual reconstructed showers. The dataset is currently too small to draw firm conclusions as-of-yet. Hence, phase I data taking will continue even as we gear up for the deployment of an Auger micro-array within Telescope Array as part of Phase II of this work.

The Auger@TA project is a combined effort involving members of both the Pierre Auger Observatory and the Telescope Array experiment (TA) to cross-calibrate detectors and compare results on air showers detected at one location. We have recently reported results from Phase I of the project, during which we collected and presented data from two Auger water Cherenkov surface detector stations deployed into the TA experiment near the Central Laser Facility. For Phase II, we will deploy a micro-array of six or seven single-PMT Auger surface detector stations co-located with TA scintillator surface detector stations. The Auger micro-array will trigger and collect data independently from the TA allowing for a complete end-to-end comparison of detector data, calibration, and reconstructed event quantities on a shower-by-shower basis between the TA and Auger detector systems. We describe progress towards development of the micro-array for Phase II including the preparation of surface detector water tanks, station electronics, wireless communications, triggers, and data acquisition. We also outline plans for deploying the Auger@TA micro-array into the TA experiment in 2019 with preliminary estimates for coincident air-shower rates.