The wavefront of the radio signal emitted by cosmic ray air showers

W D Apel ,P Łuczak,J Oehlschläger,C Rühle,J Blümer,H.J Mathes,A Schmidt,O Sima,L Bähren,M Roth,K Daumiller,C Grupen,B Fuchs,H Falcke,M Bertaina,O Krömer,J Kuijpers,А Чиавасса ,V De Souza,S Schoo,Daniel Huber ,G Toma,G Trinchero ,M Melissas,D Heck,T Huege,R Engel,N Palmieri,A Haungs,A Săftoiu ,J Rautenberg,F Di Pierro,D Kang,J Wochele,K H Kampert ,I M Brancus ,H Bozdog,P G Isar ,C Morello,H Schieler,H Rebel,K Link,J R Hörandel ,H Gemmeke,Frank G Schröder ,P L Biermann ,A Weindl,A Horneffer,K Bekk,E Cantoni,J Zabierowski,P Döll ,M Ludwig,T Pierog,J C Arteaga‐Velázquez ,J A Zensus

doi:10.1088/1475-7516/2014/09/025

Abstract

Analyzing measurements of the LOPES antenna array together with corresponding CoREAS simulations for more than 300 measured events with energy above 1017 eV and zenith angles smaller than 45̂, we find that the radio wavefront of cosmic-ray air showers is of approximately hyperbolic shape. The simulations predict a slightly steeper wavefront towards East than towards West, but this asymmetry is negligible against the measurement uncertainties of LOPES. At axis distances ≳ 50 m, the wavefront can be approximated by a simple cone. According to the simulations, the cone angle is clearly correlated with the shower maximum. Thus, we confirm earlier predictions that arrival time measurements can be used to study the longitudinal shower development, but now using a realistic wavefront. Moreover, we show that the hyperbolic wavefront is compatible with our measurement, and we present several experimental indications that the cone angle is indeed sensitive to the shower development. Consequently, the wavefront can be used to statistically study the primary composition of ultra-high energy cosmic rays. At LOPES, the experimentally achieved precision for the shower maximum is limited by measurement uncertainties to approximately 140 g/c 2. But the simulations indicate that under better conditions this method might yield an accuracy for the atmospheric depth of the shower maximum, Xmax, better than 30 g/c 2. This would be competitive with the established air-fluorescence and air-Cherenkov techniques, where the radio technique offers the advantage of a significantly higher duty-cycle. Finally, the hyperbolic wavefront can be used to reconstruct the shower geometry more accurately, which potentially allows a better reconstruction of all other shower parameters, too.

Highlights

Antenna arrays like LOFAR [2], and for sparser, large-scale arrays which can be built at reasonable costs
Analyzing measurements of the LOPES antenna array together with corresponding CoREAS simulations for more than 300 measured events with energy above 1017 eV and zenith angles smaller than 45◦, we find that the radio wavefront of cosmic-ray air showers is of approximately hyperbolic shape
We find a small asymmetry of the wavefront in the CoREAS simulations: for the geomagnetic field at the LOPES site, the wavefront is slightly steeper towards East than towards West

Summary

Introduction

Antenna arrays like LOFAR [2], and for sparser, large-scale arrays which can be built at reasonable costs. The refractive index of the air influences the coherence conditions, which leads to observable effects in the lateral distribution of the radio amplitude [10], and affects the radio wavefront All these effects are included in the CoREAS [11] Monte-Carlo-simulation code used for this paper. LOPES is a pathfinder experiment to study the principles of the radio emission and to develop analysis techniques for the radio measurements It is not a precision experiment, which already could compete with the precision of established techniques for air showers, like air-fluorescence, air-Cherenkov, or secondary particle measurements. For closer shower maxima the amplitude decreases faster with increasing distance to the shower axis This method has recently been exploited for a first reconstruction of Xmax based on measured LOPES events [17]

Results

Discussion

Conclusion