Observation of inclined EeV air showers with the radio detector of the Pierre Auger Observatory

Abstract

With the Auger Engineering Radio Array (AERA) of the Pierre Auger Observatory, we have observed the radio emission from 561 extensive air showers with zenith angles between 60 • and 84 • . In contrast to air showers with more vertical incidence, these inclined air showers illuminate large ground areas of several km 2 with radio signals detectable in the 30 to 80 MHz band. A comparison of the measured radio-signal amplitudes with Monte Carlo simulations of a subset of 50 events for which we reconstruct the energy using the Auger surface detector shows agreement within the uncertainties of the current analysis. As expected for forward-beamed radio emission undergoing no significant absorption or scattering in the atmosphere, the area illuminated by radio signals grows with the zenith angle of the air shower. Inclined air showers with EeV energies are thus measurable with sparse radio-antenna arrays with grid sizes of a km or more. This is particularly attractive as radio detection provides direct access to the energy in the electromagnetic cascade of an air shower, which in case of inclined air showers is not accessible by arrays of particle detectors on the ground.

Reconstructing the cosmic-ray energy from the radio signal measured in one single station

“…Under small viewing angles, signals from different stages in the shower's development arrive simultaneously, leading to an amplified emission over a wide frequency band [35]. The resulting dependency of the frequency spectrum on ϕ has already been observed in the 30 − 80 MHz band [23,[36][37][38] and is used to reconstruct air shower energies by the ANITA experiment [39]. The method used by ANITA, however, requires dedicated Monte Carlo simulations for each reconstructed event and is therefore not suitable for large arrays with regular detections.…”

Enhancing the cosmic-ray mass sensitivity of air-shower arrays by combining radio and muon detectors

“…Thus, the radio signal provides information about the size of the electromagnetic component for all zenith angles. Furthermore, the width of the radio footprint on ground rises with the zenith angle [14], which enables the economic detection of inclined showers with radio antenna arrays with a large spacing.…”

Targeting ultra-high energy neutrinos with the ARIANNA experiment

“…This is facilitated by a newly developed technique to determine the air-shower energy from a single radio detector station (Welling et al, 2019). Also, the radio signal of inclined air showers, where this technique works best, extends over a large area (Aab et al, 2018). Thus, a sparse spacing of detector stations of O(1 km) is sufficient to perform this measurement.…”