33ND INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013 THE ASTROPARTICLE PHYSICS CONFERENCE Calibration and Reconstruction Performance of the HAWC Observatory

Abstract: The High Altitude Water Cherenkov (HAWC) experiment is being built at an altitude of 4100 m at Sierra Negra volcano near Puebla, Mexico, to serve as an observatory for gamma-rays with energies between 50 GeV and 100 TeV. Upon completion, the array will consist of 300 water Cherenkov detectors (WCDs) each equipped with four photo-multiplier tubes (PMTs) to detect Cherenkov light from air showers passing through the experiment. For optimal reconstruction of the direction of the shower primaries, in particular to map gamma-ray events, the relative timing of each PMT must be calibrated with nanosecond (ns) precision over the full dynamic range of the PMTs. This is achieved by sending short (300 picoseconds) laser light pulses through a network of optical fibers into every detector. As the laser calibration system provides intensities from < 0.1 photo electrons (PEs) to> 1000 PEs, the charge calibration of all PMTs is also obtained and further improves the angular resolution of gamma-ray event directions. In this contribution, we present the design and performance of the laser calibration system. The improvement of air shower reconstruction due to calibration is verified for an analysis of the shadow that the moon produces in the flux of cosmic rays, based on data collected with a partial array of 30 WCDs