GZK Photons in the Ultra High Energy Cosmic Rays

We calculate the ux of GZK- photons, namely the ux of Ultra High Energy Cosmic Rays (UHECR) consisting of photons produced by extragalactic protons through the resonant photoproduction of pions, the so called Greisen-Zatsepin-Kuzmin (GZK) effect. We show that if the UHECR are mostly protons, depending on the slope of the proton ux, distribution of sources and intervening backgrounds, between 0.01 % and 5% of the UHECR above eV are photons. Detection of these photons will open new window for UHECR gamma-ray astronomy. Detection of a larger photon ux would imply the emission of photons at the source or new physics. 1