Latest results on the analysis of the radio frequency spectrum emitted by high energy air showers with LOFAR

¹ Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands
² Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
³ NIKHEF, Science Park Amsterdam, 1098 XG Amsterdam, The Netherlands
⁴ Netherlands Institute of Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo The Netherlands
⁵ KVI-CART, University Groningen, P.O. Box 72, 9700 AB Groningen The Netherlands
⁶ DESY, Platanenallee 6, 15738 Zeuthen Germany
⁷ Humboldt-Universität zu Berlin, Institut für Physik, Newtonstrasse 15, 12489 Berlin Germany
⁸ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 USA
⁹ Interuniversity Institute for High-Energy, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels Belgium
¹⁰ Department of Physics and Electrical Engineering, Linnéuniversitetet, 35195 Växjö Sweden

^★ e-mail: l.rossetto@astro.ru.nl

Published online: 24 September 2019

Abstract

The LOw Frequency ARay (LOFAR) is a multi-purpose radio antenna array aimed to detect radio signals in the frequency range 10 - 240 MHz, covering a largesurface in Northern Europe with a higher density in the Netherlands. Analytical calculations and simulation studies performed in the 2000s indicate a dependence of the radio frequency spectrum on cosmic-ray characteristics. The high number density of radio antennas at the LOFAR core allows to characterise the observed cascade in a detailed way. The radio signal emitted by air showers in the atmosphere has been studied accurately in the 30 - 80 MHz frequency range. The analysis has been conducted on simulated eventsand on real data detected by LOFAR since 2011. The final aim of this study is to find an independent method to infer information of primary cosmic rays for improving the reconstruction of primary particle parameters. Results show a strong dependence of the frequency spectrum on the distance to the shower axis for both real data and simulations. Furthermore, results show that this method is very sensitive to the precision in reconstructing the position of the shower axis at ground, and to different antenna calibration procedures. A correlation between the frequency spectrum and geometrical distance to the shower maximum development Xmax has also been investigated.