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Astron. Astrophys. 359, 337-346 (2000)

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Observations of the supernova remnant W28 at TeV energies

G.P. Rowell 1, T. Naito 6, S.A. Dazeley 2, P.G. Edwards 3, S. Gunji 4, T. Hara 6, J. Holder 1, A. Kawachi 1, T. Kifune 1, Y. Matsubara 8, Y. Mizumoto 9, M. Mori 1, H. Muraishi 10, Y. Muraki 8, K. Nishijima 7, S. Ogio 5, J.R. Patterson 2, M.D. Roberts 1, T. Sako 8, K. Sakurazawa 5, R. Susukita 11, T. Tamura 12, T. Tanimori 5, G.J. Thornton 2, S. Yanagita 10, T. Yoshida 10 and T. Yoshikoshi 1

1 Institute for Cosmic Ray Research, University of Tokyo, Tokyo 188-8502, Japan
2 Department of Physics and Math. Physics, University of Adelaide, Adelaide 5005, Australia
3 Institute of Space and Astronautical Science, Kanagawa 229-8510, Japan
4 Department of Physics, Yamagata University, Yamagata 990-8560, Japan
5 Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
6 Faculty of Management Information, Yamanashi Gakuin University, Yamanashi 400-8575, Japan
7 Department of Physics, Tokai University, Kanagawa 259-1292, Japan
8 Solar-Terrestrial Environment Lab., Nagoya University, Aichi 464-8601, Japan
9 National Astronomical Observatory of Japan, Tokyo 181-8588, Japan
10 Faculty of Science, Ibaraki University, Ibaraki 310-8512, Japan
11 Institute of Physical and Chemical Research, Saitama 351-0198, Japan
12 Faculty of Engineering, Kanagawa University, Kanagawa 221-8686, Japan

Received 19 October 1999 / Accepted 26 April 2000

Abstract

The atmospheric erenkov imaging technique has been used to search for point-like and diffuse TeV gamma-ray emission from the southern supernova remnant, W28, and surrounding region. The search, made with the CANGAROO 3.8 m telescope, encompasses a number of interesting features, the supernova remnant itself, the EGRET source 3EG J1800-2338, the pulsar PSR J1801-23, strong 1720 MHz OH masers and molecular clouds on the north and east boundaries of the remnant. An analysis tailored to extended and off-axis point sources was used, and no evidence for TeV gamma-ray emission from any of the features described above was found in data taken over the 1994 and 1995 seasons. Our upper limit ([FORMULA] TeV) for a diffuse source of radius 0.25o encompassing both molecular clouds was calculated at 6.64[FORMULA] cm-2 s-1 (from 1994 data), and interpreted within the framework of a model predicting TeV gamma-rays from shocked-accelerated hadrons. Our upper limit suggests the need for some cutoff in the parent spectrum of accelerated hadrons and/or slightly steeper parent spectra than that used here (-2.1). As to the nature of 3EG J1800-2338, it possibly does not result entirely from [FORMULA] decay, a conclusion also consistent with its location in relation to W28.

Key words: ISM: supernova remnants – ISM: individual objects: W28 – gamma rays: observations

Present address: Max Planck Institut für Kernphysik, 69029 Heidelberg, Germany

Correspondence to: Gavin.Rowell@mpi-hd.mpg.de

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© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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