Interpreting the high energy emission of Fermi GRBs

The high energy emission from Gamma-ray Bursts has some interesting features, including the absence of the GeV excess in the prompt spectrum, the delayed onset of the GeV emission, and the longer duration of the GeV emission than the prompt soft gamma-ray emission. We suggest that the non-detection of a GeV excess in most GRB spectrum may favor the magnetic fireball model and the early prompt emission may be dominated by the photosphere radiation of the breakout material and is thus very soft. The synchrotron radiation in GeV band can be the dominant component of the high energy afterglow emission, as speculated in GRB 080319B and then confirmed in GRB 080916C and GRB 090510. A simple estimate of the thermal radiation of the breakout material has been presented.