Frequency spectra of cosmic ray air shower radio emission measured with LOPES^*

¹ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands e-mail: anigl@astro.ru.nl
² Institut für Kernphysik, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
³ Institut für Experimentelle Kernphysik, Universität Karlsruhe (TH), 76021 Karlsruhe, Germany
⁴ Institut für Prozessverarb. und Elektr., Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
⁵ Fachbereich Physik, Universität Wuppertal, 42097 Wuppertal, Germany
⁶ ASTRON, 7990 AA Dwingeloo, The Netherlands
⁷ Dipartimento di Fisica Generale dell'Università, 10125 Torino, Italy
⁸ Max-Planck-Institut für Radioastronomie, 53010 Bonn, Germany
⁹ National Institute of Physics and Nuclear Engineering, 7690 Bucharest, Romania
¹⁰ Fachbereich Physik, Universität Siegen, 57068 Siegen, Germany
¹¹ Istituto di Fisica dello Spazio Interplanetario, INAF, 10133 Torino, Italy
¹² Soltan Institute for Nuclear Studies, 90950 Lodz, Poland
¹³ Physics Department, Bucharest University, Bucharest-Magurele, PO Box MG-11, RO-077125, Romania

Received: 7 December 2007
Accepted: 17 June 2008

Abstract

Aims. We wish to study the spectral dependence of the radio emission from cosmic-ray air showers around .

Methods. We observe short radio pulses in a broad frequency band with the dipole-interferometer LOPES (LOFAR Prototype Station), which is triggered by a particle detector array named Karlsruhe Shower Core and Array Detector (KASCADE). LOFAR is the Low Frequency Array. For this analysis, 23 strong air shower events are selected using parameters from KASCADE. The radio data are digitally beam-formed before the spectra are determined by sub-band filtering and fast Fourier transformation.

Results. The resulting electric field spectra fall off to higher frequencies. An average electric field spectrum is fitted with an exponential and , or alternatively, with a power law and a spectral index of . The spectral slope obtained is not consistent within uncertainties and it is slightly steeper than the slope obtained from Monte Carlo simulations based on air showers simulated with CORSIKA (Cosmic Ray Simulations for KASCADE). For the analyzed sample of LOPES events, we do not find any significant dependence of the spectral slope on the electric field amplitude, the azimuth angle, the zenith angle, the curvature radius, nor on the average distance of the antennae from the shower core position. But one of the strongest events was measured during thunderstorm activity in the vicinity of LOPES and shows the longest pulse length measured of and a spectral slope of .

Conclusions. We show with two different methods that frequency spectra from air shower radio emission can be reconstructed on event-by-event basis, with only two dozen dipole antennae simultaneously over a broad range of frequencies. According to the obtained spectral slopes, the maximum power is emitted below 40 MHz. Furthermore, the decrease in power to higher frequencies indicates a loss in coherence determined by the shower disc thickness. We conclude that a broader bandwidth, larger collecting area, and longer baselines, as will be provided by LOFAR, are necessary to further investigate the relation of the coherence, pulse length, and spectral slope of cosmic ray air showers.