Limits on the Isotropic Diffuse Flux of Ultrahigh Energy γ Radiation

Diffuse ultrahigh energy γ radiation can arise from a variety of astrophysical sources, including the interaction of 10²⁰eV cosmic rays with the 2.7 K microwave background radiation or the collapse of topological defects created in the early Universe. We describe a sensitive search for diffuse γ rays at ultrahigh energies using the Chicago Air Shower Array--Michigan Muon Array experiment. An isotropic flux of radiation is not detected, and we place stringent upper limits on the fraction of the γ-ray component relative to cosmic rays ( <10^-4) at energies from 5.7×10¹⁴ to 5.5×10¹⁶eV. This result represents the first comprehensive constraint on the γ-ray flux at these energies.