4 Origin of the line and constraints on the emitting region

It is most likely that the line of GRB 970508 is produced by fluorescence and recombination of Fe atoms ionized by the intense flux of the GRB and its afterglow (Piro et al. 1999; Lazzati et al. 1999). In the early phases of the GRB the radiation field is so high that iron atoms are completely stripped of their electrons: the Compton temperature is very high and then recombination is not very efficient in producing line photons. When the flux decreases, about 10⁴ s after the burst, fluorescence becomes an effective process. We note, in passing, that the intensity of the line is therefore not correlated with the luminosity of the burst: for example, with a luminosity a factor of 10 larger, the medium would have remained completely ionized upto about 1 day after the burst, producing therefore a line with a lower intensity. The minimum mass needed to produce the line is (Piro et al. 1999; Lazzati et al. 1999) $M_{\rm min} = 0.5\, M_\odot \,A_{\rm Fe}^{-1}$, where $A_{\rm Fe}$ is the iron abundance normalized to the solar value. From the line variability, intensity and width we deduce that this medium should be located at a distance of $\approx 3\ 10^{15}$ cm from the central source, it is moving with subrelativistic speed, it should have a large density ($n\gt 5\ 10^9\,{\rm cm}^{-3}$), and it should lie sideways respect to the observer, otherways it would smear out the short timescale structure of the burst with Thomson scattering (Boettcher et al. 1999).

 

Figure1: The spectra (in detector counts) of the afterglow of GRB 970508 in the first a) and and second part of the observation b) fitted with a power law continuum

In order to reach such distance, this material must have been pre-ejected by the source originating the burst, shortly (perhaps a year, for typical SN expansion speeds) before the burst. We stress that these observations contain two coincidences: on the one hand, this is the only burst in which a reburst and a line have been observed by BeppoSAX; on the other, the iron line disappears exactly at the moment of the reburst. We also point out that a line feature, with a similar significance, has been found by ASCA in another burst, GRB 970828, which also shows an event of rebursting during the X-ray afterglow (Yoshida et al. 1999). On the contrary, neither GRB 971214 nor GRB 98613 show rebursting (Costa 1999).

Acknowledgements

We thank the BeppoSAX team for the support with observations. BeppoSAX is a program of the Italian space agency (ASI) with the participation of the Dutch space agency (NIVR).