3 Summary

ASCA saw a fading X-ray afterglow from GRB 970828. However its flux not monotonically decayed but displayed a "flare activity'' around $t=1.25\ 10^5$ s. The BeppoSAX observed an X-ray flux variability also in the GRB afterglow of GRB 970508 (Piro et al. 1997). It is a rather broad ($\delta t \gt 10^5$ s at $t\approx10^5$ s) "bursting activity'', while the activity seen with ASCA is narrower in time; $\delta t/t\sim0.05$. Fenimore et al. (1998) suggests that this small $\delta t/t$ implies a small surface filling factor similar to the GRB activities themselves and there would be fine structures in the relativistic shell much smaller than $\Gamma^{-1}$ where $\Gamma$ is the bulk Lorentz factor.

ASCA discovered in the afterglow spectra the evidence of a large absorption column density and an emission line possibly caused by the Fe K$_{\alpha}$ line emissions red shifted by z=0.33. This would be the first GRB with its red shift determined only by the X-ray observation if this interpretation is true. A detection of a red-shifted Fe emission line is also reported by the BeppoSAX team for the GRB 970508 afterglow (Piro et al. 1999a; Piro et al. 1999b). The Fe line seen in the GRB 970828 was variable in time with the time scale of $\sim10\,000$ s and appeared $\sim10^5$ s after the burst. This suggests a rather small line forming region of $\sim10^{14}-10^{15}$ cm and implies a dense matter (n>10¹² for the matter with the solar abundance) in the vicinity of the source if the line is formed by the photoionization-recombination process or the "reflection'' mechanism (Yoshida et al. 1999).

Acknowledgements

We thank the ASCA team for allowing this TOO.