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[34.01] Low Upper limits of Iron line Flux in two X-ray afterglow of GRBs with ASCA

M. Toshio, Y. Daisuke (ISAS), Y. Atsumasa (Riken), GRB ISAS Team, GRB Collaboration

The iron emission line in the X-ray afterglow of GRBs is thought to be a key to understand the environment of progenitors. Two deep observations of X-ray agterglows can be explained by a simple power-law spectra but without any iron emission feature with ASCA. The 90% upper-limit for the iron flux is as low as only 100 eV(EW). This is an order of lower than the previous two observations.