SEARCH FOR POINT SOURCES OF ULTRAHIGH-ENERGY GAMMA-RAYS IN THE SOUTHERN-HEMISPHERE WITH THE SOUTH-POLE AIR SHOWER EXPERIMENT

We report on the results of a search for point sources of ultrahigh en ergy gamma radiation in data collected in 1988, 1990, and 1991 by the South Pole Air Shower Experiment. Nine predefined point sources were i nvestigated: the x-ray binaries SMC X-1, LMC X-4, Cen X-3, Vela X-1, 4 U1626-67, 4U1145-61, the supernova 1987A, the globular cluster 47 Tuca nae, and the unconfirmed source BL-1. No conclusive evidence was found for dc emission from any of the nine candidates. An all sky search fo r a time-averaged signal was performed, but no significant excess was found. We find a 95% C.L. flux limit of 2.0 X 10(-13) cm-2s-1 above 50 TeV for all sources, with the exception of Vela X-1 where we have set the limit at 0.6 X 10(-13) cm-2s-1 above 200 TeV. The four x-ray bina ries SMC X-1, LMC X-4, Cen X-3, and Vela X-1 were investigated for gam ma-ray emission modulated with the orbital period. No evidence for a m odulated gamma-ray signal was found. A search for sporadic emission fr om the nine sources was conducted on time scales of one hour, one day, and one week. The hourly and weekly burst searches were unsuccessful, but a statistically significant excess from SMC X-1 (99.6% C.L.) was detected during one day in 1991: 178 events on-source versus 120 backg round events. If this excess is attributed to a gamma-ray signal, the associated flux above 50 TeV is (1.3 +/- 0.2) X 10(-11) cm-2s-1.