MULTI-TeV GAMMA-RAY OBSERVATION FROM THE CRAB NEBULA USING THE TIBET-III AIR SHOWER ARRAY FINELY TUNED BY THE COSMIC RAY MOON'S SHADOW

Abstract

The Tibet-III air shower array, consisting of 533 scintillation detectors,\nhas been operating successfully at Yangbajing in Tibet, China since 1999. Using\nthe dataset collected by this array from 1999 November through 2005 November,\nwe obtained the energy spectrum of $\\gamma$-rays from the Crab Nebula,\nexpressed by a power law as $(dJ/dE) = (2.09\\pm0.32)\\times10^{-12} (E/{\\rm 3\nTeV})^{-2.96\\pm0.14} {\\rm cm}^{-2} {\\rm s}^{-1} {\\rm TeV}^{-1}$ in the energy\nrange of 1.7 to 40 TeV. This result is consistent with other independent\n$\\gamma$-ray observations by imaging air Cherenkov telescopes. In this paper,\nwe carefully checked and tuned the performance of the Tibet-III array using\ndata on the moon's shadow in comparison with a detailed Monte Carlo simulation.\nThe shadow is shifted to the west of the moon's apparent position as an effect\nof the geomagnetic field, although the extent of this displacement depends on\nthe primary energy positively charged cosmic rays. This finding enables us to\nestimate the systematic error in determining the primary energy from its shower\nsize. This error is estimated to be less than $\\pm$12% in our experiment. This\nenergy scale estimation is the first attempt among cosmic-ray experiments at\nground level. The systematic pointing error is also estimated to be smaller\nthan $0\\fdg011$. The deficit rate and position of the moon's shadow are shown\nto be very stable within a statistical error of $\\pm$6% year by year. This\nguarantees the long-term stability of point-like source observation with the\nTibet-III array. These systematic errors are adequately taken into account in\nour study of the Crab Nebula.\n