Tibet AS+MD Project

PROCEEDINGS OF THE 31st ICRC, ŁOD´ Z´ 2009 1 Tibet AS+MD Project M. Amenomori∗, X. J. Bi†, D. Chen‡, S. W. Cui§, Danzengluobu¶, L. K. Ding†, X. H. Ding¶, C. Fank, C. F. Fengk, Zhaoyang Feng†, Z. Y. Feng∗∗, X. Y. Gao††, Q. X. Geng††, Q. B. Gou†, H. W. Guo¶, H. H. He†, M. Hek, K. Hibino‡‡, N. Hottax , Haibing Hu¶, H. B. Hu†, J. Huang†, Q. Huang∗∗, H. Y. Jia∗∗, L. Jiang†† †, F. Kajinoxi , K. Kasaharaxii , Y. Katayosexiii , C. Katoxiv , K. Kawata‡, Labaciren¶, G. M. Lexv , A. F. Lik, H. C. Li§†, J. Y. Lik, C. Liu†, Y.-Q. Louxvi , H. Lu†, X. R. Meng¶, K. Mizutanixii xvii , J. Mu††, K. Munakataxiv , A. Nagaixviii , H. Nanjo∗, M. Nishizawaxix , M. Ohnishi‡, I. Ohtaxx , S. Ozawaxii , T. Saitoxxi , T. Y. Saitoxxii , M. Sakataxi , T. K. Sako‡, M. Shibataxiii , A. Shiomixxiii , T. Shirai‡‡, H. Sugimotoxxiv , M. Takita‡, Y. H. Tan†, N. Tateyama‡‡, S. Toriixii , H. Tsuchiyaxxv , S. Udo‡‡, B. Wang†, H. Wang†, Y. Wang†, Y. G. Wangk, H. R. Wu†, L. Xuek, Y. Yamamotoxi , C. T. Yanxxvi , X. C. Yang††, S. Yasuexxvii , Z. H. Yexxviii , G. C. Yu∗∗, A. F. Yuan¶, T. Yuda‡‡, H. M. Zhang†, J. L. Zhang†, N. J. Zhangk, X. Y. Zhangk, Y. Zhang†, Yi Zhang†, Ying Zhang∗∗ †, Zhaxisangzhu¶ and X. X. Zhou∗∗ (The Tibet ASγ Collaboration) ∗Department of Physics, Hirosaki University, Hirosaki 036-8561, Japan. †Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. ‡Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582, Japan. §Department of Physics, Hebei Normal University, Shijiazhuang 050016, China. ¶Department of Mathematics and Physics, Tibet University, Lhasa 850000, China. kDepartment of Physics, Shandong University, Jinan 250100, China. ∗∗Institute of Modern Physics, SouthWest Jiaotong University, Chengdu 610031, China. ††Department of Physics, Yunnan University, Kunming 650091, China. ‡‡Faculty of Engineering, Kanagawa University, Yokohama 221-8686, Japan. x Faculty of Education, Utsunomiya University, Utsunomiya 321-8505, Japan. xi Department of Physics, Konan University, Kobe 658-8501, Japan. xii Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan. xiii Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan. xiv Department of Physics, Shinshu University, Matsumoto 390-8621, Japan. xv National Center for Space Weather, China Meteorological Administration, Beijing 100081, China. xvi Physics Department and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, China. xvii Saitama University, Saitama 338-8570, Japan. xviii Advanced Media Network Center, Utsunomiya University, Utsunomiya 321-8585, Japan. xix National Institute of Informatics, Tokyo 101-8430, Japan. xx Sakushin Gakuin University, Utsunomiya 321-3295, Japan. xxi Tokyo Metropolitan College of Industrial Technology, Tokyo 116-8523, Japan. xxii Max-Planck-Institut fur¨ Physik, Munchen¨ D-80805, Deutschland. xxiii College of Industrial Technology, Nihon University, Narashino 275-8576, Japan. xxiv Shonan Institute of Technology, Fujisawa 251-8511, Japan. xxv RIKEN, Wako 351-0198, Japan. xxvi Institute of Disaster Prevention Science and Technology, Yanjiao 065201, China. xxvii School of General Education, Shinshu University, Matsumoto 390-8621, Japan. xxviii Center of Space Science and Application Research, Chinese Academy of Sciences, Beijing 100080, China. Abstract. We are proposing a future project to ob- can significantly discriminate between the primary serve 10-1000 TeV gamma rays from the cosmic-ray gamma rays and cosmic-ray background events by accelerators in our Galaxy with very low background means of counting the number of muons in an air noise and wide field of view. In order to achieve this shower. Our current MC simulation predicts that the purpose, we are planning to build a 10,000 m2 water- cosmic-ray background events can be suppressed by Cherenkov-type muon detector (MD) array under the approximately 99.99% at 100 TeV using a full-scale Tibet air shower (AS) array. The gamma-ray induced MD array. air shower has significantly less muons compared Keywords: Gamma rays, Muons, Cerenkov detec- with a cosmic- ray induced one. Therefore, we tors. 2 M. AMENOMORI et al. TIBET AS+MD PROJECT I. INTRODUCTION 1,700 Hz. Using these arrays, we already successfully Recent imaging atmospheric Cherenkov telescopes detected VHE gamma rays from Crab, Mrk 501 and (IACTs), such as CANGAROO, H.E.S.S., MAGIC and Mrk 421 [8][9][10]. Also, we set stringent upper limits VERITAS, and wide field-of-view extensive air shower to gamma rays from Galactic plane at 3 TeV and 10 TeV arrays, such as Milagro and the Tibet air shower (Ti- [11][12]. bet AS) array, detect a lot of very-high-energy (VHE) Recently, we updated the VHE gamma-ray energy gamma-ray sources in the 100 GeV to 10 TeV en- spectrum of the Crab Nebula [13]. We found no evidence ergy region. Remarkably, H.E.S.S. reported dozens of for time variability of flux intensity from the Crab new unidentified TeV gamma-ray sources which have a Nebula at multi-TeV energies. Also the VHE gamma- harder energy spectrum (indices: −1.8 to −2.8) at TeV ray energy spectrum of the Crab Nebula is consistent energies than the standard candle Crab (index: −2.6) [1]. with our previous results and other observations by These are called “ dark cosmic-ray accelerator”. the IACTs. We also have successfully observed VHE Cosmic rays are supposed to be accelerated up to the gamma-ray sources and precise large-scale cosmic-ray knee energy region (∼4 PeV) [2] at VHE astrophysical anisotropy in the northern sky [14][15]. In these results, objects in our galaxy. Therefore, we naturally expect we first pointed out new small anisotropies in Cygnus gamma rays in the 100 TeV region which originate in region at multi-TeV energies [15]. One of them is π0 decays produced by the accelerated charged cosmic coincident with MGRO J2019+37 which the Milagro rays interacting with matter surrounding VHE gamma- experiment recently established as a VHE gamma-ray ray source. However, the origin of VHE gamma rays source [16]. might also be attributed to the leptonic processes, such as The absolute gamma-ray energies in multi-TeV region the inverse-Compton (IC) scattering or bremsstrahlung observed by the Tibet AS array are verified by the by accelerated electrons. One solution of this dilemma Moon’s shadow observation [13]. As primary cosmic is to find a peak of π0 decays around 70 MeV in the rays are shielded by the Moon, we observed a deficit in gamma-ray energy spectrum. The FERMI gamma-ray cosmic rays called the Moon’s shadow, and the center space telescope, succeeding the EGRET telescope, was of the Moon’s shadow shifts westward depending on launched on June 2008 to cover an energy range from primary cosmic-ray energies due to the geomagnetic 20 MeV to 300 GeV [3], with a sensitivity approximately field. Using this effect, the systematic error of absolute hundred times better than EGRET. An alternative ap- energy scale is estimated to be less than ±12%. proach is to search for a continuous gamma-ray spectrum III. TIBET MD ARRAY up to 100 TeV or more, because gamma rays of the leptonic origin rapidly diminished at higher energies A problem in VHE gamma-ray astronomy with the due to the strong synchrotron cooling process and the ground-based experiment is the presence of dominant Klein-Nishina effect. Although the air shower experi- cosmic-ray background events against gamma-ray sig- ments have [4][5][6] searched for gamma-ray sources in nals. In order to significantly discriminate between the 100 TeV region, there is no compelling evidence gamma rays and background cosmic rays, we are for gamma-ray sources so far. Thus, it is interesting planning to build a water-Cherenkov-type muon de- to observe the gamma-ray sources above 10 TeV to tector array (Tibet MD array) around the Tibet AS investigate the mechanism of gamma-ray emissions, the array [17][18][19][20], because gamma-ray induced air cosmic-ray acceleration and its origins. shower have much less muons than cosmic-ray induced In this paper, we will introduce our future plan which one. Each muon detector is a waterproof concrete pool, is called the Tibet AS+MD project. The Tibet AS+MD 7.2 m wide × 7.2 m long × 1.5 m deep in size, equipped array will have the sensitivity to gamma rays in the with two 20 inch-in-diameter PMT (HAMAMATSU 100 TeV region by an order of magnitude better than R3600) as shown in Fig. 1. The Tibet MD array consists previous existing detectors. of 192 muon detectors set up 2.5 m underground as shown by the shaded areas in Fig. 2. Its total effective II. TIBET AS ARRAY area amounts to be 10,000 m2 approximately for muon The Tibet AS experiment has been successfully op- detection with an energy threshold of 1 GeV. The surface erated at Yangbajing (90.522◦ E, 30.102◦ N, 4,300 m AS array will be a simple extension of the Tibet AS above sea level) in Tibet, China since 1990 [7]. The array array, consisting of 1081 scintillation detectors covering was constructed first in 1990 and gradually upgraded by an effective area of 83,000 m2. increasing the number of detectors, then the Tibet AS The Monte Carlo simulation including the air shower array consists of 533 plastic scintillation detectors of generation and responses of the Tibet AS+MD array 0.5 m2 placed on a 7.5 m square grid to detect high- was done to estimate the discrimination power between energy ( > a few TeV) cosmic-ray showers. In the late gamma rays and cosmic-ray background events based fall of 2003, the area of the Tibet AS array was further on counting the number of muons accompanying an enlarged up to 36,900 m2 by adding 256 detectors.

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