A search for TeV gamma-ray emission from the southern SNR W28 was carried out by the CANGAROO over two observation seasons (1994 and 1995) using the atmospheric erenkov imaging technique. An analysis providing a consistent gamma-ray acceptance and quality factor for extended sources was used. A number of sites within a search region of were considered as potential point-like and diffuse sources of TeV gamma-ray emission. No evidence was found for the emission of TeV gamma-rays at any of these sites which include those from the strongest two masers, an EGRET source, a radio pulsar (all as point sources) and a diffuse region containing the molecular clouds that appear to be interacting with the remnant. Our 3 upper limit from this diffuse region at 6.64 cm-2 s-1 TeV, and the flux of the EGRET source 3EG J1800-2338 were compared with gamma-ray flux predictions from the model of Naito & Takahara (1994). Under this framework, our upper limit rules out a straight extrapolation of the EGRET flux to TeV energies. It also constrains somewhat the flux expected from the shocked region of gas in the molecular cloud, placing limits on the parent spectra for hadrons and/or cutoff energy. Our results suggest that the EGRET source probably does not result entirely of gamma-rays. This fact is supported by it's location in relation to the molecular clouds. In a later paper, we will consider electronic bremsstrahlung and inverse Compton scattering and discuss the broader implications of our results in relation to the origin of galactic cosmic rays.
Further data on W28 will no doubt be taken with the recently completed CANGAROO II telescope (Yoshikoshi et al. 1999), which, at the very least will provide tighter constraints on models of gamma-ray production for this interesting source.
© European Southern Observatory (ESO) 2000
Online publication: June 30, 2000