Search for point sources of ultrahigh energyγrays in the southern hemisphere with the South Pole Air Shower Experiment

We report on the results of a search for point sources of ultrahigh energy $\gamma$ radiation in data collected in 1988, 1990, and 1991 by the South Pole Air Shower Experiment. Nine predefined point sources were investigated: the x-ray binaries SMC X-1, LMC X-4, Cen X-3, Vela X-1, 4U1626-67, 4U1145-61, the supernova 1987A, the globular cluster 47 Tucanae, and the unconfirmed source BL-1. No conclusive evidence was found for dc emission from any of the nine candidates. An all sky search for a time-averaged signal was performed, but no significant excess was found. We find a 95% C.L. flux limit of 2.0×${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ above 50 TeV for all sources, with the exception of Vela X-1 where we have set the limit at 0.6×${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ above 200 TeV. The four x-ray binaries SMC X-1, LMC X-4, Cen X-3, and Vela X-1 were investigated for $\gamma$-ray emission modulated with the orbital period. No evidence for a modulated $\gamma$-ray signal was found. A search for sporadic emission from 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 background events. If this excess is attributed to a $\gamma$-ray signal, the associated flux above 50 TeV is (1.3±0.2)×${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$.