Iron line signatures in X-ray afterglows of GRB by BeppoSAX

¹ Istituto di Astrofisica Spaziale, C.N.R., Roma, Italy
² Istituto T.E.S.R.E., C.N.R., Bologna, Italy
³ Dipartimento di Fisica, Universita' di Ferrara, Italy
⁴ BeppoSAX Science Data Center, Rome, Italy
⁵ Osservatorio Astronomico di Roma, Italy
⁶ Space Research Organization in the Netherlands, Utrecht, The Netherlands
⁷ Space Science Department of ESA, ESTEC
⁸ Istituto Fisica Cosmica e Appl. Calc. Informatico, C.N.R., Palermo, Italy
⁹ Dipartimento di Fisica, Universita' Roma Tre, Roma, Italy

Received: 16 March 1999
Accepted: 25 June 1999

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 GRB 970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. 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. We did not find evidence of Fe line in two other GRB with known redshift (GRB 971214 and GRB 980613), but we note that the upper limits are of the same order of the intensity measured in GRB 97508 and that none of these afterglows shows rebursting activity.