Measurement of the Lateral Distribution Function of UHECR Air Showers with the Pierre Auger Observatory

The Pierre Auger Observatory [1] is being used to study cosmic rays with energies larger than 10 eV with unprecedented precision and statistics. An essential quantity that must be deduced from data is the lateral distribution function (LDF) that describes the decreasing of the signals in the watertanks as a function of distance. Knowledge of the LDF is important for the reconstruction of the shower core and the shower direction. It can also be compared with model calculations to give useful information relating to primary mass. Here we describe how the LDF is measured using the large sample of events recorded with the surface detector (SD) array and with a small sample observed with the fluorescence detectors (FD). For hybrid events, in which SD and FD measurements of the same shower are available, the core position is much better constrained than for SD-only events, thus providing an important cross-check on the LDF determined from SD measurements alone.