The shape of the radio wavefront of extensive air showers as measured with LOFAR

A Corstanje, P Schellart, A Nelles, S Buitink, J E Enriquez, H Falcke, W Frieswijk, J R Hörandel, M Krause, J P Rachen, O Scholten, S ter Veen, S Thoudam, T N G Trinh, M van den Akker, A Alexov, J Anderson, I M Avruch, M E Bell, M J BentumG Bernardi, P Best, A Bonafede, F Breitling, J Broderick, M Brüggen, H R Butcher, B Ciardi, F de Gasperin, E de Geus, M de Vos, S Duscha, J Eislöffel, D Engels, R A Fallows, C Ferrari, Michael Garrett, J Grießmeier, A W Gunst, J P Hamaker, M Hoeft, A Horneffer, M Iacobelli, E Juette, A Karastergiou, J Kohler, V I Kondratiev, M Kuniyoshi, G Kuper, P Maat, G Mann, R McFadden, D McKay-Bukowski, M Mevius, H Munk, M J Norden, E Orru, H Paas, M Pandey-Pommier, V N Pandey, R Pizzo, A G Polatidis, W Reich, H Röttgering, A M M Scaife, D Schwarz, O Smirnov, A Stewart, M Steinmetz, J Swinbank, M Tagger, Y Tang, C Tasse, C Toribio, R Vermeulen, C Vocks, R J van Weeren, S J Wijnholds, O Wucknitz, S Yatawatta, P Zarka

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    Extensive air showers, induced by high energy cosmic rays impinging on the Earth’s atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parameterization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle.
    Original languageEnglish
    Pages (from-to)22-31
    Number of pages10
    JournalAstroparticle Physics
    Publication statusPublished - 2015


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