OG 4 . 2 - 1 A STUDY OF CYG X - 3 , HER X - 1 , AND THE CRAB NEBULA FOR GAMMA RAY EMISSIONS ABOVE 7 x

A portion of the Fly's Eye II has been recording Cherenkov flashes since November, 1987, in a declination band which includes these three x-ray sources. The data have been analyzed on a nightly basis in a search for evidence of ultra-high energy (UHE) gamma ray emissions. Introduction. Cyg X-3, Her X-1, and the Crab nebula or pulsar have all been reported to be sources of UHE ã-rays. Reviews on Cyg X-3 are Protheroe (1987) and Bonnet-Bidaud and Chardin (1988). Her X-1 reports include Baltrusaitis et al. (1985) and Dingus at al. (1988). UHE emissions from the Crab were reported in Boone et al. (1983) and Dzikowski et al. (1981). We here report results from Cherenkov flash data recorded by the Fly's Eye II detector at Dugway, Utah. The data for each source have been analyzed on a night-by-night basis. Data acquisition. In order to avoid interfering with the Fly's Eye's principal role of recording scintillation tracks from higher energy showers, the Cherenkov flash data rate has been restricted. Only 9 of the 36 mirror units in Fly's Eye II are set to trigger if a strong pulse is recorded in a single photomultiplier tube (PMT). Those 9 mirrors observe a band of declinations which includes the three candidate sources. The threshold for single-tube triggers is set high in order to limit the detector's dead time and also to avoid triggering on noise from background light. For each trigger, data is stored from every Fly's Eye II PMT which exceeded a low threshold in any of 3 channels with different integration times. Cherenkov flashes are distinguished from airplane strobes, lightning, and other spurious triggers by virtue of their narrow time profile, so the amplitude in the fastest channel is such higher than the amplitude in the slowest channel. Some Cherenkov flashes are detected by more than one PMT. Any PMT is included in the event if it reached threshold almost simultaneously with the highest-amplitude PMT, and if it is contiguous with other PMT's in the event. For each Cherenkov flash event, summary information is stored for subsequent analysis. The stored information includes the number of PMT's in the event, the total photoelectron amplitude, the time of trigger, and the central direction of the PMT with the largest amplitude. (The field of view for each PMT has a radius of 2.6°.) For each night, a check is made on the relative rate stability in the different mirror units. Bad weather conditions or the onset of twilight can cause severe rate deviations in one or more mirrors. The criteria for acceptable rate behavior have been specified (Baltrusaitis et al. 1987). From 11-Nov-1987 through 11-Jul-1989, a total of 5.29x10 Cherenkov flashes have been recorded during acceptable running conditions on 219 nights. The accepted running time amounts to 4.23x10 seconds. The solid angle covered by the 9 mirror units is 0.75 sr.