The very early afterglow powered by the ultra-relativistic mildly   magnetized outflows
  In the Poynting Flux dominated outflow (the initial ratio of the electromagnetic energy flux to the particle energy flux $\sigma_0\gg1$) model for Gamma-ray bursts, nearly half of the internally dissipated magnetic energy is converted into the prompt $\gamma-$ray energy emission and the rest is converted into the kinetic energy of the outflow. Consequently, at the end of the $\gamma-$ray burst, $\sigma$ decreases significantly ($\sigma\sim 1$ or even smaller). We numerically investigate the very early reverse shock emission powered by such mildly magnetized outflows interacting with medium--uniform interstellar medium (ISM) or stellar wind (WIND). We show that for $\sigma\sim0.05-1$ and typical parameters of Gamma-ray bursts, both the ISM-ejecta interaction and the WIND-ejecta interaction can power very strong optical emission ($m_{\rm R}\sim 10-12{\rm th}$ magnitude or even brighter). Similar to the very early afterglow powered by the non-magnetized ejecta interacting with the external medium, the main difference between the ISM-ejecta interaction case and the WIND-ejecta interaction case is that, before the reverse shock crosses the ejecta, the R-band emission flux increases rapidly for the former, but for the latter it increases only slightly. (The abstract has been shortened). We suggest that the linear polarization detection of the early multi-wavelength afterglow is highly needed to see whether the outflows powering GRBs are magnetized or not. 
Y. Z. Fan, D. M. Wei, C. F. Wang