Transient Absorption Features in Gamma-Ray Bursts and Their Implications for Gamma-Ray Burst Progenitors

The recent detection of a transient absorption feature in the prompt emission of GRB 990705 has sparked multiple attempts to fit this feature in terms of photoelectric absorption or resonance scattering out of the line of sight to the observer. However, the physical conditions required to reproduce the observed absorption feature turn out to be rather extreme compared to the predictions of current gamma-ray burst (GRB) progenitor models. In particular, strong clumping of ejecta from the GRB progenitor seems to be required. Using detailed three-dimensional hydrodynamic simulations of supernova explosions as a guideline, we have investigated the dynamics and structure of pre-GRB ejecta predicted in various GRB progenitor models. Based on our results, combined with population synthesis studies relevant to the He-merger model, we estimate the probability of observing X-ray absorption features as seen in GRB 990705 to ≪1%. Alternatively, if the supranova model is capable of producing highly collimated, long-duration GRBs, it may be a more promising candidate to produce observable, transient X-ray absorption features.