The spectrum of Gamma-ray Burst: a clue

摘要

In this work we numerically calculate the thermal radiation efficiency of thebaryonic outflow. The possible outflow acceleration in the transparent stage,which lowers thermal radiation efficiency, has been taken into account. In thestandard internal shock model for the prompt emission, the fast shells shouldmove with a typical Lorentz factor $\gtrsim 5 \Gamma_{\rm i}$ otherwise the GRBefficiency will be in disagreement with the observations, where $\Gamma_{\rmi}$ is the bulk Lorentz factor of the shocked/emitting region. The photosphereradius of these fast shells is small and the thermal radiation is too strong tobe effectively outshone by the internal shock emission. This is particularlythe 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 inthe spectrum of most GRBs challenges the standard internal shock model and maysuggest a non-baryonic (magnetic) outflow component. Though the magneticoutflow model seems favored by more and more data, it can hardly reproduce thetypical GRB spectrum. In the photosphere-gradual magnetic dissipation scenario,the spectrum cuts off at $\sim 1$ GeV, too low to account for the observationsof GRBs 080916C. In the sudden magnetic energy dissipation model, the lowenergy spectrum is expected to be $F_\nu \propto \nu^{-1/2}$, too soft to beconsistent with the data $F_\nu \propto \nu^{0}$. We speculate that the lowenergy spectrum puzzle could be unveiled by the mechanism that particles, inthe magnetic dissipation process, are repeatedly accelerated.