Are the Ultra‐Energetic Cosmic Gammas Really Photons?

A renaissance of cosmic‐ray physics in this decade has been doling out tantalizing bits of evidence for extraordinary phenomena involving photons far more energetic than one can make in the laboratory. A 1983 report of observations at the University of Kiel's extensive‐air‐shower array argued that the mysterious x‐ray and radio source Cygnus X‐3 was sending us photons of energy exceeding $1015$ electron volts (1000 TeV) from half way across the Galaxy. Cosmic photons of more than a few TeV had never before been seen to come from “point sources.” Nor could such energies be explained by conventional models of cosmic‐ray acceleration. Furthermore, it appeared that these ultrahigh‐energy photons, when they hit the upper atmosphere, were behaving very differently from the confident expectations of particle theory.