A Possible Emission Feature in an X-Ray Afterglow of GRB 970828 as a Radiative Recombination Edge

A gamma-ray burst (GRB) of 1997 August 28 was localized by the All-Sky Monitor on the Rossi X-Ray Timing Explorer satellite, and its coordinates were promptly disseminated. An ASCA follow-up started 1.17 days after the burst as a Target-of-Opportunity Observation and detected an X-ray afterglow. The spectral data displayed a hump around ~5 keV and an absorption column of 7.1 × 1021 cm-2. Taking into account the redshift z = 0.9578 found for the likely host galaxy of the associated radio flare, this hump structure is likely a recombination edge of iron in the vicinity of the source. The radiative recombination edge and continuum model can interpret the spectrum from highly ionized plasma in a nonequilibrium ionization state. The absorption could also be due to the medium presumably residing in the vicinity of the GRB.