The X-ray afterglow of the gamma-ray burst of 1997 May 8: Spectral variability and possible evidence of an iron line

We report the possible detection (99.3% of statistical significance) of red shifted iron line emission in the X-ray afterglow of gamma-ray burst GRB 97 0508 observed by BeppoSAX. Its energy is consistent with the redshift of th e putative host galaxy determined from optical spectroscopy. The line disap peared similar to 1 day after the burst. We have also analyzed the spectral variability during the outburst event that characterizes the X-ray aftergl ow of this gamma-ray burst. The spectrum gets harder during the fare, then becoming steep when the flux decreases. The variability, intensity, and wid th of the line indicate that the emitting region should have a mass greater than or similar to 0.5 M-. (assuming that the iron abundance is similar to its solar value), should have a size of similar to 3 x 10(15) cm, is distr ibuted anisotropically, and is moving with subrelativistic speed. In contra st to the fairly clean environment expected in the merging of two neutron s tars, the observed line properties would imply that the site of the burst i s embedded in a large mass of material, consistent with preexplosion ejecta of a very massive star. This material could be related with the outburst o bserved in the afterglow 1 day after the GRB and with the spectral variatio ns measured during this phase.