New results of the digital radio interferometer LOPES

Abstract

LOPES was a digital, phased antenna array located at the site of KASCADE-Grande in Karlsruhe, Germany, operating from 2003 to 2013. Triggered by the particle detector array of KASCADE, LOPES measured the radio signal of air showers between 40 and 80MHz (effective bandwidth: 43-74MHz). By cross-correlation beamforming, i.e., an interferometric, offline combination of the signals measured at different antennas, LOPES was able to enhance the signal-to-noise ratio. This lowered the detection threshold significantly close to 0:1EeV, despite the high human-made radio background at the experimental site. Although LOPES already has been dismantled, data analysis is still continued. Recent progress concerns a new amplitude calibration of the existing data, described in the second proceeding, and the incorporation of detector effects in air-shower simulations produced with CoREAS. These end-to-end simulations enable a better comparison of measurements with theoretical predictions. Furthermore, the new end-to-end simulations have been used to study the effect of the antenna and the bandpass filter on the slope of the lateral distribution and the shape of the wavefront. Both parameters can be used to reconstruct the shower maximum independently: the reconstruction of the lateral distribution relies on amplitude measurements, and the reconstruction of the hyperbolic radio wavefront relies on time measurements. In the new end-to-end simulations, the lateral distributions are slightly steeper than in the previously used simplified simulations. For the wavefront we do not find a significant change, but with both methods, the uncertainties are still large due to the high background at the LOPES site. Nevertheless, with the end-to-end simulations we do find a large influence of the used method for reconstruction of the hyperbolic wavefront: Fitting pulse arrival times in individual antennas, the reconstructed cone angle of the wavefront is on average half of the angle determined by cross-correlation beamforming. This makes a quantitative comparison between different experiments and to simulations more difficult, since all details of the wavefront reconstruction have to be considered appropriately. Nevertheless, the two effects on the wavefront reconstruction (cross-correlation beamforming vs. arrival time fit; simplified vs. end-to-end simulations) on average cancel out, such that previously published LOPES results remain valid. In particular the cone angle r of the wavefront predicted by CoREAS is compatible with the measured one.