On the absorption feature in the prompt X-ray spectrum of GRB 990705

The absorption feature detected in the prompt X-ray emission of GRB 990705 bears important consequences. We investigate different production mechanism s, and we conclude that the absorbing material cannot be very close to the burster and is likely to be moderately clumped. These properties challenge any model in which the burst explodes in coincidence with the core collapse of a massive rotating star. We show that the straightforward interpretatio n of the absorption feature as a photoionization K edge of neutral iron fac es a severe problem in that it requires a huge amount of iron in the close vicinity of the burster. We then discuss an alternative scenario in which i ron ions are kept in a high-ionization state by the burst flux and the abso rption feature is produced by resonant scattering from hydrogen-like iron b roadened by a range of outflow velocities. In this case, the physical condi tions and geometry of the absorbing material are fully consistent with the presence of a young supernova remnant surrounding the burst site at a radiu s R similar to 10(16) cm. We finally discuss how this remnant might affect the generation of afterglows with a standard power-law flux decay.