Results of a southern-hemisphere search for gamma-ray sources at E/sub gamma/greater than or equal to3times1011 eV

The optical intensity interferometer operated by Sydney University at Narrabri, NSW, Australia, was converted in 1972 April to a sensitive detector of atmospheric Cerenkov light from extensive air showers (EAS) with E0>2times1011 eV. Its two 7-m aperture optical reflectors were separated by 120 m and pointed by computer at the mean altitude for the maximum development of EAS from the direction of an object to be studied. Off-axis photomultipliers on the reflectors viewed the atmospheric Cerenkov light from EAS muon cores, thus enabling rejection of greater than or equal to50 percent of EAS detected as being cosmic-ray-initiated. Possible (greater than or equal to3 sigma) sources were found in 1972 in the directions of Cen A (NGC 5128), the Vela Pulsar, and MP 1451, out of 11 candidate objects studied. The objects included pulsars, unusual X-ray sources, active galaxies, a supernova, and lower energy (approx.100 MeV) γ-ray sources such as the galactic center. (Confirmation of the 1972 results was attempted in an improved series of observations in 1973 April-June and 1974 March-April. The Cen A result was confirmed and is reported elsewhere. Upper limits are given here for the other objects, together with evidence for a variable for a variable pulsed flux from the Vela Pulsar. Implications of the results are discussed