Ultra-high-energy gamma rays observed by the Fly's Eye

Besides observing tracks of distant 1017to 1020eV air showers, the Fly's Eye can observe Cherenkov flashes from nearby 1013 to 1016eV showers. These have been used to search for ultra-high-energy point sources. Weak evidence of a signal has been observed from the vicinity of the Crab pulsar. Signals from Cygnus X-3 and Hercules X-1 have been detected by observing periodicity in the data. In the case of Cygnus X-3, spectral information has also been obtained. Evidence was obtained for sporadic emission from Cygnus X-3 including emission near phase 0 of the 4.8 hour period. Hercules X-1 was observed at a time when the X-ray beam was inferred to be absorbed by obscuring matter. The Hercules X-1 signal requires the acceleration to occur near the pulsar in the binary system. The production of such energetic γ-rays from Hercules X-1 is a challenge for acceleration models