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Volume 358 - 36th International Cosmic Ray Conference (ICRC2019) - GRI - Gamma Ray Indirect
ALPACA air shower array to explore 100TeV gamma-ray sky in Bolivia
T. Sako,* C. Calle, K. Hibino, N. Hotta, Y. Katayose, C. Kato, S. Kato, K. Kawata, W. Kihara, Y. Ko, H. Kojima, R. Mayta, P. Miranda, H. Mitsui, K. Munakata, H. Nakada, Y. Nakamura, M. Nishizawa, S. Ogio, M. Ohnishi, T. Ohura, A. Oshima, M. Raljevich, H. Rivera, T. Saito, T.K. Sako, Y. Sengoku, S. Shibata, A. Shiomi, M. Subieta, N. Tajima, W. Takano, M. Takita, Y. Tameda, K. Tanaka, R. Ticona, H. Tsuchiya, Y. Tsunesada, S. Udo, K. Yagisawa, Y. Yokoe
*corresponding author
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Pre-published on: 2019 July 22
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Abstract
Andes Large area PArticle detector for Cosmic ray physics and Astronomy (ALPACA) is a new air shower array project as a collaboration between Bolivia and Japan to explore the 100 TeV gamma-ray sky in the southern hemisphere. In a plateau near the Chacaltaya mountain at 4,740 m altitude, a surface detector array covering 82,800 m$^{2}$ with underground water Cherenkov muon detectors of total 5,400 m$^{2}$ area will be constructed. Because of 2 m soil overburden, the muon detectors can detect muons of >1.2 GeV in air showers with a high purity. Using the conventional surface array to determine the primary energy and the arrival direction, the underground muon detectors improve the gamma/hadron separation and also mass identification of primary cosmic rays. For gamma-ray showers within zenith angle of 45 degrees, ALPACA has a full effective area above 20TeV. At 20 TeV and 100 TeV, 99% and 99.9% hadron showers are rejected, respectively, while keeping the gamma-ray detection efficiency above 90%. Many interesting galactic objects can be observed with 0.2 degree angular resolution at 100 TeV with >2,000 hours/year exposure. ALPACA enables us the first sensitive survey of the southern gamma-ray sky at 100 TeV energy range that is crucial to identify PeV accelerating objects. Preparation for infrastructure and con- struction of a pathfinder array ALPAQUITA are ongoing. Scientific targets, expected performance of ALPACA including the prospects for some CR observations and current status are described.
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