  We have modeled the simultaneous first-order Fermi shock acceleration of
protons, electrons, and helium nuclei by relativistic shocks. By parameterizing
the particle diffusion, our steady-state Monte Carlo simulation allows us to
follow particles from particle injection at nonthermal thermal energies to
above PeV energies, including the nonlinear smoothing of the shock structure
due to cosmic-ray (CR) backpressure. We observe the mass-to-charge (A/Z)
enhancement effect believed to occur in efficient Fermi acceleration in
non-relativistic shocks and we parameterize the transfer of ion energy to
electrons seen in particle-in-cell (PIC) simulations. For a given set of
environmental and model parameters, the Monte Carlo simulation determines the
absolute normalization of the particle distributions and the resulting
synchrotron, inverse-Compton, and pion-decay emission in a largely
self-consistent manner. The simulation is flexible and can be readily used with
a wide range of parameters typical of gamma-ray burst (GRB) afterglows. We
describe some preliminary results for photon emission from shocks of different
Lorentz factors and outline how the Monte Carlo simulation can be generalized
and coupled to hydrodynamic simulations of GRB blast waves. We assume Bohm
diffusion for simplicity but emphasize that the nonlinear effects we describe
stem mainly from an extended shock precursor where higher energy particles
diffuse further upstream. Quantitative differences will occur with different
diffusion models, particularly for the maximum CR energy and photon emission,
but these nonlinear effects should be qualitatively similar as long as the
scattering mean free path is an increasing function of momentum.
