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Received; accepted

By L. Piro, E. Costa, M. Feroci, F. Frontera, L. Amati, D. Dal Fiume, J. Heise, J. In ’t Z, A. Owens, A. N. Parmar, G. Cusumano, M. Vietri and G. C. Perola

Abstract

We report the possible detection (99.3 % of statistical significance) of redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. The line disappeared ∼ 1 day after the burst. We have also analyzed the spectral variability during the outburst event that characterizes the X-ray afterglow of this GRB. The spectrum gets harder during the flare, turning to steep when the flux decreases. The variability, intensity and width of the line indicate that the emitting region should have a mass ∼> 0.5M ⊙ (assuming the iron abundance similar to its solar value), a size of ∼ 3 × 1015cm, is distributed anisotropically, and is moving with sub-relativistic speed. In contrast to the fairly clean environment expected in the merging of two neutron stars, the observed line properties would imply that the site of the burst is embedded in a large mass of material, consistent with pre-explosion ejecta of a very massive star. This material could be related with the outburst observed in the afterglow 1 day after the GRB and with the spectral variations measured during this phase

Year: 1999
OAI identifier: oai:CiteSeerX.psu:10.1.1.317.6153
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