2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/96149We 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.To appear in Proc. of 22nd Texas Symposium on Relativistic Astrophysics, Stanford Univ., December 2004, (PSN 2205), 5 pages, LaTeX, 6 eps figuresAstrophysicsThe very early optical afterglow of GRBs, revealing the nature of the ejectatext