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

The Tibet-III air shower array, consisting of 533 scintillation detectors, has been operating successfully at Yangbajing in Tibet, China since 1999. Using the dataset collected by this array from 1999 November through 2005 November, we obtained the energy spectrum of γ-rays from the Crab Nebula, expressed by a power law as (dJ/dE) = (2.09± 0.32)× 10(E/3 TeV)−2.96±0.14cm−2s−1TeV in the energy range of 1.7 to 40 TeV. This result is consistent with other independent γ-ray observations by imaging air Cherenkov telescopes. In this paper, we carefully checked and tuned the performance of the Tibet-III array using data on the moon’s shadow in comparison with a detailed Monte Carlo simulation. The shadow is shifted to the west of the moon’s apparent position as an effect of the geomagnetic field, although the extent of this displacement depends on the primary energy positively charged cosmic rays. This finding enables us to estimate the systematic error in determining the primary energy from its shower size. This error is estimated to be less than ±12% in our experiment. This energy scale estimation is the first attempt among cosmic-ray experiments at ground level. The systematic pointing error is also estimated to be smaller than 0. 011. The deficit rate and position of the moon’s shadow are shown to be very stable within a statistical error of ±6% year by year. This guarantees the long-term stability of point-like source observation with the Tibet-III array. These systematic errors are adequately taken into account in our study of the Crab Nebula. Subject headings: cosmic rays — gamma rays : observations — magnetic fields — Moon — pulsars : individual (Crab pulsar) — supernova remnants : individual (Crab Nebula) 1 Department of Physics, Hirosaki University, Hirosaki 036-8561, Japan. 2 Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. 3 Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582, Japan. 4 Department of Physics, Hebei Normal University, Shijiazhuang 050016, China. 5 Department of Mathematics and Physics, Tibet University, Lhasa 850000, China. 6 Department of Physics, Shandong University, Jinan 250100, China. 7 Institute of Modern Physics, SouthWest Jiaotong University, Chengdu 610031, China. 8 Department of Physics, Yunnan University, Kunming 650091, China. 9 Faculty of Engineering, Kanagawa University, Yokohama 2218686, Japan. 10 Faculty of Education, Utsunomiya University, Utsunomiya 321-8505, Japan. 11 Department of Physics, Konan University, Kobe 658-8501, Japan. 12 Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan. 13 Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan. 14 Department of Physics, Shinshu University, Matsumoto 3908621, Japan.