Improved absolute calibration of LOPES measurements and its impact on the comparison with REAS 3.11 and CoREAS simulations

TL;DR

This paper revises the calibration of LOPES radio measurements of cosmic-ray air showers, leading to a significant amplitude scale correction that aligns experimental data with CoREAS simulations but not REAS 3.11.

Contribution

It provides an improved absolute calibration for LOPES measurements, significantly altering previous amplitude results and their comparison with simulation models.

Findings

LOPES amplitude scale decreased by a factor of 2.6

Revised calibration makes LOPES data compatible with CoREAS simulations

Discrepancy remains with REAS 3.11 simulations

Abstract

LOPES was a digital antenna array detecting the radio emission of cosmic-ray air showers. The calibration of the absolute amplitude scale of the measurements was done using an external, commercial reference source, which emits a frequency comb with defined amplitudes. Recently, we obtained improved reference values by the manufacturer of the reference source, which significantly changed the absolute calibration of LOPES. We reanalyzed previously published LOPES measurements, studying the impact of the changed calibration. The main effect is an overall decrease of the LOPES amplitude scale by a factor of $2.6 \pm 0.2$, affecting all previously published values for measurements of the electric-field strength. This results in a major change in the conclusion of the paper 'Comparing LOPES measurements of air-shower radio emission with REAS 3.11 and CoREAS simulations' published in Astroparticle Physics 50-52 (2013) 76-91: With the revised calibration, LOPES measurements now are compatible with CoREAS simulations, but in tension with REAS 3.11 simulations. Since CoREAS is the latest version of the simulation code incorporating the current state of knowledge on the radio emission of air showers, this new result indicates that the absolute amplitude prediction of current simulations now is in agreement with experimental data.