The very early optical afterglow of GRBs, revealing the nature of the   ejecta
  We show that if the GRB ejecta itself is magnetized or neutron-rich, the very early afterglow of GRBs is very different from that powered by a pure ion fireball. In the Swift era, with the well monitored early afterglow data, we can potentially diagnose the ejecta composition and reveal the nature of the central GRB engine. 
Y. Z. Fan, Bing Zhang and D. M. Wei