GeV Photons from Ultrahigh Energy Cosmic Rays Accelerated in Gamma Ray Bursts

Abstract

$\gamma$-ray bursts are produced by the dissipation of the kinetic energy of a highly relativistic fireball, via the formation of a collisionless shock. When this happens, ultrahigh energy cosmic rays up to $\approx {10}^{20}\mathrm{eV}$ are produced. I show in this Letter that these particles produce, via synchrotron emission as they cross the acceleration region, photons up to 300 GeV, which carry away a small, $\approx 0.01$, but non-negligible fraction of the total burst energy. I show that, when the shock occurs with the interstellar medium, the optical depth to photon-photon scattering, which might cause energy degradation of the photons, is small. Expected fluences, $\approx {10}^{-5\mathrm{}}–{10}^{-6\mathrm{}}\mathrm{erg}{\mathrm{cm}}^{-2\mathrm{}}$, are within the range of planned detectors.