AN IRON-ABSORPTION MODEL OF GAMMA-RAY BURST SPECTRA

Most gamma-ray bursts (GRBs) exhibit deficits of X-rays below approximately 200 keV. Here we consider a spectral model in which the burst source is shielded by an optically thick layer of circumburster material (CBM) rich in Fe-group elements whose photoelectric absorption opacity exceeds the Thomson opacity below approximately 120 keV. For power-law distributions of absorption depths along the lines of sight the absorbed spectrum can indeed mimic the typical GRB spectrum. This model predicts that (a) the spectrum should evolve monotonically from hard to soft during each energy release, which is observed in most bursts, especially in fast rise exponential decay bursts; (b) Fe spectral features near 7 keV may be present in some bursts; and (c) the ratio of burst distances to the CBM and to Earth should be approximately 10(-11) if the spectral evolution is purely due to Fe stripping by the photons.