Electromagnetic Catastrophe in Ultrarelativistic Shocks and the Prompt Emission of Gamma-Ray Bursts

It is shown that an ultrarelativistic shock with the Lorentz factor of order of tens or higher propagating in a moderately dense interstellar medium (density above $\sim 1000 $ cm$^{-3}$) undergoes a fast dramatic transformation into a highly radiative state. The process leading to this phenomenon resembles the first order Fermi acceleration with the difference that the energy is transported across the shock front by photons rather than protons. The reflection of the energy flux crossing the shock front in both directions is due to photon-photon pair production and Compton scattering. Such mechanism initiates a runaway nonlinear pair cascade fed directly by the kinetic energy of the shock. Eventually the cascade feeds back the fluid dynamics, converting the sharp shock front into a smooth velocity gradient and the runaway evolution changes to a quasi-steady state regime. This effect has been studied numerically using the nonlinear Large Particle Monte-Carlo code for the electromagnetic component and a simplified hydrodynamic description of the fluid. The most interesting application of the effect is the phenomenon of gamma-ray bursts where it explains a high radiative efficiencyand gives a perspective to explain spectra of GRBs and their time variability. The results predict a phenomenon of ``GeV bursts'' which arise if the density of the external medium is not sufficiently high to provide a large compactness