A SURVEY of possible celestial gamma ray sources was undertaken by the detection of Cerenkov light generated in the atmosphere by energetic particles in air showers. Upper limits for thirteen possible continuous sources of gamma rays of energy greater than 2 × 1012 eV, together with a description of the system and its operation, have been reported elsewhere1. Soon after the discovery of the pulsating radio sources2, the survey was extended to include observations of these objects. The results are presented here together with some preliminary observations of the same sources made with a new instrument with larger aperture (10 m) and lower threshold energy (4 × 1011 eV). Most of the observations were made at the 4,200 foot level of the Mount Hopkins observing station near Tucson, Arizona, by means of light detectors consisting of two 5 foot searchlight mirrors with RCA 4518 photomulipliers at their foci; these were operated in coincidence with a resolving time of 6 ns. The results of eleven drift scans on three of the pulsating radio sources are shown in Table 1. The location of CP 0950 near Leo XR-1 permitted an upper limit to be obtained for the former from re-examination of the previous drift scans on the latter. The upper limits to the gamma ray fluxes in Table 1 are at the three standard deviation level of significance.
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