Upper limits of iron line flux in X-ray afterglow of two Gamma-Ray Bursts with ASCA

The iron emission line in the X-ray afterglow spectra of a Gamma-Ray Burst is thought to be a key to understanding the environment of progenitors. Alt hough two independent observations with BeppoSAX and ASCA reported strong r ed-shifted iron emission line, the statistical significance of these detect ions was not very high. Further observations were expected and two more GRB s, GRB 990123 and GRB 990704, were observed with the X-ray satellite ASCA i n 1999. The spectra of these two afterglows can be explained by a simple po wer-law model with excess absorption, but without any iron emission-line fe ature. The 90% upper limit for the iron line flux is as low as F-Fe = 3.3 x 10(-6) photons cm(-2) s(-1), or less than similar to 100 eV in equivalent width (EW). This upper limit is an order-of-magnitude smaller than the prev ious two detections of 1-3 keV in EW.