Measurement of the lateral distribution function of radio emission from ultra high energy cosmic ray induced air showers

The Radio Detector of the Pierre Auger Observatory recorded 492 air shower events in coincidence with the Auger Surface Detector from April 2007 till May 2008. We investigate the lateral distribution function (LDF) which describes the variation of the radio signal with different positions of the observer and the shower axis. As observable we choose the relative difference of the radio signal intensity observed in single air shower events with the two logarithmic-periodic dipole antennas of the setup. We find that this observable is sensitive to the LDF when observed as a function of distance D between the antennas and the shower axis. Using Monte-Carlo techniques to estimate the impact of experimental uncertainties on our measurements we find that a purely exponential shape of the LDF is unlikely to explain the measurements. We observe that a better agreement can be achieved with an LDF model that exhibits a lateral falloff ∝1/Dk with k ∼1.2.