Limits on the Isotropic Diffuse Flux of Ultrahigh Energy<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi></mml:math>Radiation

Abstract

Diffuse ultrahigh energy $\ensuremath{\gamma}$ radiation can arise from a variety of astrophysical sources, including the interaction of ${10}^{20}\mathrm{eV}$ 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 $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$-ray component relative to cosmic rays ( $&lt;{10}^{\ensuremath{-}4}$) at energies from $5.7\ifmmode\times\else\texttimes\fi{}{10}^{14}$ to $5.5\ifmmode\times\else\texttimes\fi{}{10}^{16}\mathrm{eV}$. This result represents the first comprehensive constraint on the $\ensuremath{\gamma}$-ray flux at these energies.