First Data of the 3000 km${}^2$ Radio Detector at the Pierre Auger Observatory

Abstract

In this contribution, we present the status and first data from the Radio Detector (RD) at the Pierre Auger Observatory, consisting of $1660$ radio antennas deployed across the $3000$ km${}^2$ surface detector array. These antennas measure the radio emission from extensive air showers in the $30-80$ MHz band, enabling electromagnetic energy measurements for air showers with zenith angles above $65\mathrm{deg}$. Combined with the muonic measurements from the water-Cherenkov detectors (WCDs), this allows mass composition studies at the highest energies. The large-scale deployment of the RD began in November 2023 and was completed in November 2024. A full end-to-end calibration shows consistency between Galactic and in-lab calibration to better than $5$\% and includes continuous monitoring for hardware failures, ensuring, for example, antenna alignment within $5\mathrm{deg}$. We present the first data, demonstrating a strong correlation between the electromagnetic energy measured by the RD and the total shower energy measured by the WCD, confirming that the detector chain - including triggering, data readout, absolute calibration, and reconstruction is well understood. We highlight a particularly impressive $32$ EeV shower at a zenith angle of $85\mathrm{deg}$, producing a $50$ km-long radio footprint, showcasing the unique capabilities of this detector.

Comments: Presented at the 39th International Cosmic Ray Conference (ICRC 2025). 8 pages, 7 figures