On the optimum distance for signal-energy mapping in extensive air shower arrays

In high energy cosmic rays surface arrays, the primary energy is currently determined from the value of the lateral distribution function (LDF) at a fixed optimum distance ropt from the shower core. The value of ropt is mainly related to the geometry of the array and is, therefore, considered as fixed independently of the shower energy or direction. Here, we demonstrate that the dependence of ropt on energy and zenith angle is not negligible. As a consequence, this method may not reconstruct properly the shape of the spectrum and might change the position of spectral features like the ankle. This result applies for pure surface array experiments but not to hybrid experiments, like the Pierre Auger Observatory, where hybrid events are used to calibrate the reconstructed energy of surface events. Additionally, events with saturated stations must be either properly corrected or eliminated altogether from the analysis when applying the ropt method. We show that calculating a specific ropt on a shower-to-shower bases, instead of using a fixed value, may avoid the problem of events with saturated stations and improve, at the same time, the reconstruction of spectral features. 0 G. Ros: Tel/fax: +34-918854954/42. E-mail address: [email protected]. In collaboration with Universidad Complutense de Madrid and Universidad Nacional Autónoma de México. Preprint submitted to Elsevier 2 August 2009