The spectrum of Gamma-ray Burst: a clue
  In this work we numerically calculate the thermal radiation efficiency of the
baryonic outflow. The possible outflow acceleration in the transparent stage,
which lowers thermal radiation efficiency, has been taken into account. In the
standard internal shock model for the prompt emission, the fast shells should
move with a typical Lorentz factor $\gtrsim 5 \Gamma_{\rm i}$ otherwise the GRB
efficiency will be in disagreement with the observations, where $\Gamma_{\rm
i}$ is the bulk Lorentz factor of the shocked/emitting region. The photosphere
radius of these fast shells is small and the thermal radiation is too strong to
be effectively outshone by the internal shock emission. This is particularly
the case for some extremely bright events having $\Gamma_{\rm i} \sim 10^{3}$,
like GRBs 080319B and 080916C. The absence of a distinct thermal component in
the spectrum of most GRBs challenges the standard internal shock model and may
suggest a non-baryonic (magnetic) outflow component. Though the magnetic
outflow model seems favored by more and more data, it can hardly reproduce the
typical GRB spectrum. In the photosphere-gradual magnetic dissipation scenario,
the spectrum cuts off at $\sim 1$ GeV, too low to account for the observations
of GRBs 080916C. In the sudden magnetic energy dissipation model, the low
energy spectrum is expected to be $F_\nu \propto \nu^{-1/2}$, too soft to be
consistent with the data $F_\nu \propto \nu^{0}$. We speculate that the low
energy spectrum puzzle could be unveiled by the mechanism that particles, in
the magnetic dissipation process, are repeatedly accelerated.
