Og 4.3.5 a Search for Evidence of Point Sources in the Cherenkov Flash Data from Fly's Eye Ii

A portion of the Fly's Eye II detector has been recording Cherenkov flashes since 1987 in the declination band 10° < ä < 50°. This band includes Cyg X-3, Her X-1, the Crab nebula, and Cyg X-1. The threshold energy is approximately 70 TeV. Observed and expected shower counts are available for all points in this declination band on a nightly basis. Searching for steady, transient, or sporadic point sources has yielded no positive detections. Flux upper limits are reported. Detection and analysis methods By triggering on a high amplitude pulse in any single phototube, the Fly's Eve II detector has been accumulating a large data base of air shower Cherenkov flashes. This has been analyzed in a search for ã-ray point sources. The cumulative results allow a search for an excess do flux from any point of the exposed sky. By examining results from the individual nights separately, we have searched for significant transient sources (bursts). To search for sporadic ã-ray sources, we have looked for sky locations exhibiting excesses of moderate significance on multiple nights. Properties of Cherenkov flash detections at Fly's Eye II were reported in some detail at the Adelaide ICRC (1J. The operating conditions have not changed. The solid angle of Cherenkov flash sensitivity is 0.75 sr and provides exposure to declinations 10° < ä < 50°. The energy threshold is approximately 70 TeV, and the effective detection area is π x 10cm. The analyzed data include 393 nights from 11 November 1987 through 17 April 1991. The running time is 8.2 x 10 seconds, and the number of detected showers is 10.1 x 10. The sky search employs a grid of 14400 points in the declination range 11° ä 50° with gridpoints separated by 1° in both declination and right ascension. For each gridpoint, the number of observed showers and the number of expected showers are determined for each night. The procedures are the same as for the candidate source study previously reported (1). A shower is counted as "observed" at a gridpoint if the gridpoint was separated from the phototube's central direction by less than 3.5° at the time of detection. The expected number of showers is computed for each gridpoint ):y averaging the observed numbers of showers at that point from 30 simulation data sets. A simulation data set is derived from the night's actual data set by changing each shower's time of detection to another time randomly sampled from the other times of shower detections. To search for persistent ã-ray sources, results from the various nights are combined by finding, for each gridpoint, the cumulative number of observed showers N and the cumulative number of expected showers X. Exposure is not uniform in right ascension or declination, but the numbers N and X are between 20000 and 40000 for the various gridpoints. The deviation ó = ( N – X ) / X can be used to measure the statistical significance of the excesses ( ó > 0 ) and deficits ( ó < 0 ).