The Status of Ultra-High-Energy Gamma-Ray Observations

In 1983 Samorski and Stamm found evidence of ultra-high-energy (UHE) gamma-ray emission from Cygnus X-3 in archival data from the Kiel air shower array. The emission appeared to be emitted only at one phase in the binary orbit. The Kiel array was sensitive to air showers generated by particles with energies above 3 × 1015 eV. Their discovery led to searches in archival data from other arrays for evidence of UHE emission from Cygnus X-3 and other potential sources in the Galaxy, and to the construction of new air shower arrays dedicated to UHE astronomy.In the mid 1980s many groups around the world claimed detections of Cygnus X-3, Hercules X-1, Vela X-1, and other objects. Cygnus X-3 is strongly variable at all wavelengths, and so it would be surprising if this variability did not extend to UHE gamma-rays also. This indeed appeared to be the case, and in the mid to late 1980s the UHE gamma-ray output showed a steady decline apart from transient UHE emission following giant radio flares. With the exception of possible detections above 1017 eV by the Fly’s Eye and Akeno Array, I am unaware of any statistically compelling claimed detections of this source reported over the past 4 years. The new arrays—CASA-MIA and SPASE—have not detected any of the claimed sources, and it seems likely that at least some of these early claims may have been premature. However, although none of the observations taken alone was of extremely high statistical significance, the observations of Cygnus X-3 by different groups showed a consistency which I feel cannot be ascribed to chance alone. The observations of Cygnus X-3 and the Crab Nebula/Pulsar are reviewed with particular reference to the results published during the last 5 years.Subject headings: gamma rays: observations — pulsars: individual (Crab Nebula) — stars: individual (Cygnus X-3)