  The typical spectrum of the prompt emission of gamma-ray bursts (GRBs)
indicates that the electron cooling is suppressed in spite of the strong
magnetic field in the standard synchrotron model. Recent Particle-in-Cell
simulations show that the particle acceleration by magnetic reconnection in a
magnetically dominated plasma can lead to small pitch angles especially in
low-energy region. Such a small pitch angle prevents electrons from cooling via
synchrotron radiation. In this paper, taking into account the effects of the
synchrotron cooling and the adiabatic cooling, we numerically calculate the
synchrotron spectra with anisotropic electron distributions. If we require a
Poynting flux larger than $10^{50}~\mbox{erg}~\mbox{s}^{-1}$ as the model is
motivated by magnetic reconnection, the bulk Lorentz factor of $\sim 1000$ and
the electron minimum Lorentz factor of $\gamma_{\rm min}\sim 10^4$ are required
to reproduce the typical GRB spectrum.

High Energy Astrophysical Phenomena