Lorenzo Amati,^1* Filippo Frontera,¹² Mario Vietri,³ Jean J. M. in 't Zand,⁴ Paolo Soffitta,⁵ Enrico Costa,⁵ Stefano Del Sordo,⁶ Elena Pian,¹ Luigi Piro,⁵ Lucio A. Antonelli,⁷ D. Dal Fiume,¹ Marco Feroci,⁵ Giangiacomo Gandolfi,⁵ Cristiano Guidorzi,² John Heise,⁴ Erik Kuulkers,⁴ Nicola Masetti,¹ Enrico Montanari,² Luciano Nicastro,⁶ Mauro Orlandini,¹ Eliana Palazzi¹

We report the discovery of a transient equivalent hydrogen column density with an absorption edge at ~3.8 kiloelectron volts in the spectrum of the prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be satisfactorily modeled with a photoelectric absorption by a medium located at a redshift of ~0.86 and with an iron abundance of ~75 times the solar one. The transient behavior is attributed to the strong ionization produced in the circumburst medium by the GRB photons. The high iron abundance points to the existence of a burst environment enriched by a supernova along the line of sight. The supernova explosion is estimated to have occurred about 10 years before the burst. Our results agree with models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova event.

¹ Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy.
² Dipartimento Fisica, Università di Ferrara, Via Paradiso 12, 44100 Ferrara, Italy.
³ Dipartimento Fisica, Terza Università di Roma, Via della Vasca Navale 84, 00146 Roma, Italy.
⁴ Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, Netherlands.
⁵ Istituto di Astrofisica Spaziale, CNR, Via Fosso del Cavaliere, 00133 Roma, Italy.
⁶ Istituto di Fisica Cosmica con Applicazioni all' Informatica, Via U. La Malfa, 153, 90146 Palermo, Italy.
⁷ Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Roma, Italy.
^*   To whom correspondence should be addressed E-mail: amati{at}tesre.bo.cnr.it