Evidence of TeV gamma-ray emission from the nearby starburst galaxy NGC 253
Faculty of Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan
2 Institute for Cosmic Ray Research, Univ. of Tokyo, Kashiwa, Chiba 277-8582, Japan e-mail: email@example.com
3 Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
4 MSSSO, Australian National University, ACT 2611, Australia
5 Department of Physics and Math. Physics, University of Adelaide, SA 5005, Australia
6 Institute for Space and Aeronautical Science, Sagamihara, Kanagawa 229-8510, Japan
7 Department of Physics, Yamagata University, Yamagata, Yamagata 990-8560, Japan
8 Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
9 Faculty of Management Information, Yamanashi Gakuin University, Kofu,Yamanashi 400-8575, Japan
10 Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
11 Department of Physics, Konan University, Hyogo 658-8501, Japan
12 Faculty of Engineering, Shinshu University, Nagano, Nagano 380-8553, Japan
13 STE Laboratory, Nagoya University, Nagoya, Aichi 464-8601, Japan
14 National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
15 Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences, Ibaraki 300-0394, Japan
16 Department of Physics, Osaka City University, Osaka, Osaka 558-8585, Japan
Accepted: 29 September 2006
Aims.The CANGAROO-II telescope observed sub-TeV gamma-ray emission from the nearby starburst galaxy NGC 253. The emission region was extended with a radial size of 0.3–0.6 degree. On the contrary, HESS could not confirm this emission and gave upper limits at the level of the CANGAROO-II flux. In order to resolve this discrepancy, we analyzed new observational results for NGC 253 by CANGAROO-III and also assessed the results by CANGAROO-II.
Methods.Observation was made with three telescopes of the CANGAROO-III in October 2004. We analyzed three-fold coincidence data by the robust Fisher Discriminant method to discriminate gamma ray events from hadron events.
Results. The result by the CANGAROO-III was negative. The upper limit of gamma ray flux was 5.8% Crab at 0.58 TeV for point-source assumption. In addition, the significance of the excess flux of gamma-rays by the CANGAROO-II was lowered to less than 4 sigma after assessing treatment of malfunction of photomultiplier tubes.
Key words: gamma rays: observations / galaxies: starburst / galaxies: individual: NGC 253 / galaxies: halos
© ESO, 2007