Measurement of the Sky Photon Background Flux at the Auger Observatory

The sky photon background flux has been measured at the southern Auger site in Malargue, Argentina, using the observatory's fluorescence detectors (FD). The analysis is based on ADC variances of pixels not triggered by the First Level Trigger. Photon fluxes are calculated for each individual pixel at each telescope. The statistics from each night of data taking allows a study of local variations in the photon flux. Results show a clear dependence of the flux on elevation angle. Time variations, possibly related to different atmospheric conditions, do not mask this dependence. In particular the flux excess above the horizon shows a rather stable and reproducible behaviour with elevation. Correlation of this dependence with atmospheric parameters can be of interest as it offers the promise of extracting those parameters directly from FD data, thus allowing cross checks with independent methods based on different monitoring devices.