Yoshikoshi, T

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29th International Cosmic Ray Conference Pune (2005) 5, 343346Performance of the Imaging Atmospheric Cherenkov Telescope Systemof CANGAROO-IIIT. Yoshikoshi , Y. Adachi , A. Asahara, G.V. Bicknell, R.W. Clay, Y. Doi,P.G. Edwards, R. Enomoto , S. Gunji, S. Hara , T. Hara  , T. Hattori, Sei. Hayashi,Y. Higashi, R. Inoue, C. Itoh	, S. Kabuki, F. Kajino, H. Katagiri, A. Kawachi,S. Kawasaki , T. Kifune, R. Kiuchi , K. Konno, L.T. Ksenofontov , H. Kubo,J. Kushida, Y. Matsubara, Y. Mizumoto  , M. Mori , H. Muraishi  , Y. Muraki, T. Naito  ,T. Nakamori, D. Nishida, K. Nishijima, M. Ohishi , J.R. Patterson, R.J. Protheroe,Y. Sakamoto, M. Sato, S. Suzuki  , T. Suzuki  , D.L. Swaby, T. Tanimori, T. Tanimura,G.J. Thornton, K. Tsuchiya , S. Watanabe, T. Yamaoka, M. Yamazaki, S. Yanagita,T. Yoshida, M. Yuasa and Y. Yukawa (a) Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan(b) Department of Physics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan(c) Research School of Astronomy and Astrophysics, Australian National University, ACT 2611, Australia(d) Department of Physics and Mathematical Physics, University of Adelaide, SA 5005, Australia(e) Department of Physics, Yamagata University, Yamagata, Yamagata 990-8560, Japan(f) Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510, Japan(g) Faculty of Management Information, Yamanashi Gakuin University, Kofu, Yamanashi 400-8575, Japan(h) Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan(i) Department of Physics, Konan University, Kobe, Hyogo 658-8501, Japan(j) Ibaraki Prefectural University of Health Sciences, Ami, Ibaraki 300-0394, Japan(k) Faculty of Engineering, Shinshu University, Nagano, Nagano 480-8553, Japan(l) Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Aichi 464-8602, Japan(m) National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan(n) School of Allied Health Sciences, Kitasato University, Sagamihara, Kanagawa 228-8555, Japan(o) Faculty of Science, Ibaraki University, Mito, Ibaraki 310-8512, JapanPresenter: T. Yoshikoshi (tyoshiko@icrr.u-tokyo.ac.jp), jap-yoshikoshi-T-abs1-og27-posterThe imaging atmospheric Cherenkov telescope system of CANGAROO-III has been in full operation withfour 10 meter diameter telescopes since March 2004 near Woomera, South Australia. The system is usedto study gamma-ray astrophysics at sub-TeV energies in the southern hemisphere. The performance of thestereoscopic system such as angular and energy resolutions for gamma rays has been investigated using MonteCarlo simulations, which are based on various calibration results, e.g., calibration of light collection efficiencyusing muon ring images. We present the estimated performance of the CANGAROO-III system as well as thereliability of our simulation code, comparing simulation results with observed data.1. IntroductionThe atmospheric Cherenkov technique has dramatically improved the signal-to-noise ratio in detecting VeryHigh Energy (VHE,  TeV) gamma rays by utilizing the imaging technique, which was first demonstratedby the Whipple 10 meter telescope in 1989 detecting gamma rays from the Crab Nebula at the 9  level [1].Sensitivity to VHE gamma rays has further been improved by the HEGRA group by adding the technique ofstereoscopic observations [2], in which two major backgrounds, local muon events and hadronic shower events,are greatly reduced owing to their locality (muons) and poor reconstruction accuracy due to more irregular344 T. Yoshikoshi et al.shower patterns (hadronic showers). Another important advantage of the stereoscopic system compared to asingle Imaging Atmospheric Cherenkov Telescope (IACT) is its ability to reconstruct air shower parameterssuch as the arrival direction and the core location more accurately. As a result, we can expect to have betterangular and energy resolutions using stereoscopic observations.The CANGAROO team, which has played a pioneering role in the southern hemisphere, has been observingthe southern sky since 1992, first with a 3.8 meter telescope equipped with a fine pixel camera (CANGAROO-I[3]) and, since March 2004, with a stereoscopic system (CANGAROO-III). Study of the total performance ofthe stereoscopic system is still underway and some preliminary results are presented here.2. CANGAROO-III Stereoscopic IACT SystemThe CANGAROO-III stereoscopic IACT system is located near Woomera, South Australia. It consists of four10 meter diameter IACTs, which are placed at the corners of a diamond shape with a 100 meter span. Data isacquired if any two telescopes are triggered within 650 ns [4] and raw trigger rates in this condition are about20 Hz. The details of the optical system, the imaging cameras and the data acquisition system are describedelsewhere [5, 6, 7]. The global (stereoscopic) trigger system has been in operation since December 2004 andthe performance summarised here is for the previous two telescope system (T2 and T3) operated with thelocal trigger mode, in which the telescopes generate triggers individually. Most of the CANGAROO-III resultsreported at this conference are obtained with this older configuration.3. Simulations0204060801001201401600 20 40 60 80 100120140160180200Number of Events0501001502002503000 20 40 60 80 100120140160180200Size/Arc-Length (p.e./degree)Figure 1. ‘Size / arc-length’ distributions forT2 (top) and T3 (bottom). The solid andhatched histograms are obtained from observeddata and simulations, respectively.Our Monte Carlo simulation code consists of the following threeparts: 1) primary particle injection and secondary particle gen-eration in the atmosphere, 2) Cherenkov light emission from theparticles and tracing it to the imaging cameras, and 3) response ofthe electronics. The air shower generation is based on GEANT3[8] and the target atmosphere is approximated by uniform 80 lay-ers of the same atmospheric depth, densities of which are calcu-lated using the U.S. standard atmosphere. Only Rayleigh scatter-ing is incorporated as a scattering process for Cherenkov photonsin the atmosphere. Light collection efficiency of the telescopeincluding electronics response is adjusted on the basis of the re-sults from muon ring analysis, which is described in the followingsubsection. In the simulations used here, gamma rays of variousenergies are injected into the atmosphere inside of a circular areaof the 500 meter radius at random.3.1 Muon Ring AnalysisThe data taken with the local trigger mode are used in the muonring analysis. First, the hit pattern of each event is fitted to anarc. Then, relatively fine ring images satisfying the followingconditions are selected: 1) number of hit pixels  , 2) arc-lengths  and 3) ﬀﬂﬁ normalized by the pixel size and dividedPerformance of the Imaging Atmospheric Cherenkov Telescope System of CANGAROO-III 345by the number of hit pixels ﬃ ! . The curvature distribution of the selected events has a single peak around0.75 "$#&%' )(+* and purity of muon ring events is high enough in this calibration (note that the curvature is notused in the above event selection). The solid histograms in Figure 1 are distributions of ‘size / arc-length’of the selected events. This amount is an indicator of the light collection efficiency of a telescope. Thesimulation code has been tuned with this result as well as with the results of optical measurements such asmirror reflectivity. The hatched histograms in Figure 1 are obtained using the simulation code and systematicuncertainty of the collection efficiency is at the 5% level.4. Performance of the System in the Current AnalysesThe angular and energy resolutions are estimated using the simulation code described above. We assume apoint source at the center of the field of view with a power-law spectral index of ,-. ! . The simulated data isanalyzed using the same procedures as for the SN 1006 data presented at this conference [9].4.1 Angular ResolutionThe arrival direction of a stereoscopic event is simply estimated as the intersection of major axes of individualCherenkov images. Figure 2 shows distributions of reconstructed arrival directions in the field of view. X andY projections of the distribution are well fitted by Lorentzians and here we define the angular resolution asFWHM obtained from the Lorentzian fit, which is /0 132546/73 //98 for gamma rays vertically injected into theatmosphere. The resolution is almost constant up to the zenith angle of 3/$ and gradually gets worse at zenithangles greater than that. X (deg.)-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Y (deg.)-2-1.5-1-0.500.511.5201020304050Arrival Direction hAD_pxEntries  3257Mean   0.02351RMS    0.4292 / ndf 2χ  98.99 / 88Prob   0.1987p0        0.218± 6.234 p1        0.0043± 0.1522 p2        0.003656± 0.006603  X (deg.)-2 -1.5 -1 -0.5 0 0.5 1 1.5 2Counts050100150200250300X Projection hAD_pyEntries  3257Mean   0.01612RMS    0.4081 / ndf 2χ  68.23 / 93Prob   0.9749p0        0.194± 5.835 p1        0.004± 0.142 p2        0.003499± 0.002649  Y (deg.)-2 -1.5 -1 -0.5 0 0.5 1 1.5 2Counts050100150200250300Y ProjectionFigure 2. Distribution of reconstructed arrival directions obtained using the gamma-ray simulations. The middle and righthistograms are X and Y projections of the left distribution, respectively.4.2 Energy ResolutionAt present, we conservatively use the mean image size of an event taken by the two telescopes as an energyestimator. The left of Figure 3 shows the distribution of mean sizes of simulated vertical gamma-ray events asa function of input energy. Here we also assume that the mean size is proportional to the gamma-ray primaryenergy on average and the line in the figure is a result of the proportional fit. The distribution of relative errorsof estimated energies is shown in the right and the average energy resolution is estimated to be about 35% byfitting a Gaussian to the distribution. Again, the resolution is almost flat up to the zenith angle of 3/: .346 T. Yoshikoshi et al.) (GeV)0 Log(E2 2.5 3 3.5 4 4.5 5 Log(Mean Size) (p.e.)11.522.533.544.550510152025Mean Size Vs Energy hREEEntries  3257Mean   0.02829RMS    0.4214 / ndf 2χ  286.4 / 69Prob       0Constant  3.4± 133.6 Mean      0.007482± -0.009128 Sigma     0.0062± 0.3544 0) / E0 (E - E-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Counts020406080100120140160180Relative Error of EnergyFigure 3. Distribution of mean sizes as a function of input energy (left). The energy estimation function is simply obtainedas a proportional t of the distribution and the distribution of relative errors of estimated energies is shown in the right.5. Development in the FutureThe energy resolution is expected to be improved by correcting the core distance dependence of Cherenkovlight intensity. A preliminary analysis with this correction indicates that the energy resolution is improved by atleast 10%. As a different point of view, the distance between a Cherenkov image and the source in the field ofview is correlated with the core distance and effective use of the distance parameter is also under investigationas well as effective elimination of the camera edge effect.6. AcknowledgementsThis work is supported by Grants-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports,Science and Technology, Japan, and the Australian Research Council. The receipt of JSPS Research Fellow-ships is acknowledged. We also thank the Defense Support Center Woomera and BAE Systems Australia.References[1] T.C. Weekes et al., ApJ, 342, 379 (1989).[2] F.A. Aharonian et al., ApJ, 539, 317 (2000).[3] T. Hara et al., NIM A 332, 300 (1993).[4] K. Nishijima et al., 29th ICRC (Pune), in these proceedings (2005).[5] A. Kawachi et al., Astropart. Phys., 14, 261 (2001).[6] S. Kabuki et al., NIM A 500, 318 (2003).[7] H. Kubo et al., 28th ICRC (Tsukuba), 5, 2863 (2003).[8] GEANT - Detector Description and Simulation Tool, CERN Program Library Long Writeup W5013(1993).[9] T. Tanimori et al., 29th ICRC (Pune), in these proceedings (2005).

Performance of the Imaging Atmospheric Cherenkov Telescope System of CANGAROO-III

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