Observations of PeV γ-rays.

Nineteen observations of ultra-high energy (about 10 to the 15th eV) gamma-rays are briefly described. Observations have been made of Cygnus X-3, the Crab Nebula, Vela X-1, Hercules X-1, LMC X-4, and Centaurus X-3. Most observations are of low statistical significance, and some of the detections may be spurious. A strong case can now be made for emission from Cygnus X-3, with 11 observations by 8 different research groups. The possible abundant production of low energy muons in the showers of gamma-ray origin is under investigation. Improvements of detector sensitivities by about 2 orders of magnitude appear to be possible. Studies of unexpected muon production, particle acceleration mechanisms, galactic cosmic ray production, new galactic and extragalactic sources, and gamma-ray absorption by 3 K photons may be possible with the improved detector systems.