Observations of the supernova remnant W28 at TeV energies

(Astron. Astrophys., in press)

G. P. Rowell, T. Naito, S. A. Dazeley, P. G. Edwards, S. Gunji, T. Hara, J. Holder, A. Kawachi, T. Kifune, Y. Matsubara, Y. Mizumoto, M. Mori, H. Muraishi, Y. Muraki, K. Nishijima, S. Ogio, J. R. Patterson, M. D. Roberts, T. Sako, K. Sakurazawa, R. Susukita, T. Tamura, T. Tanimori, G. J. Thornton, S. Yanagita, T. Yoshida, T. Yoshikoshi

The atmospheric Cerenkov 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 3.8m 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 (E>1.5 TeV) for a diffuse source of radius 0.25deg encompassing both molecular clouds was calculated at 6.64e-12 photons 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 pi-zero decay, a conclusion also consistent with its location in relation to W28.