Transient Absorption Features in GRBs and Their Implications for GRB 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 GRB progenitor models. In particular, strong clumping of ejecta from the GRB progenitor seems to be required. Using detailed 3D 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.