AN IRON-ABSORPTION MODEL OF GAMMA-RAY BURST SPECTRA

Most gamma-ray bursts (GRBs) exhibit deficits of X-rays below approxim ately 200 keV. Here we consider a spectral model in which the burst so urce is shielded by an optically thick layer of circumburster material (CBM) rich in Fe-group elements whose photoelectric absorption opacit y exceeds the Thomson opacity below approximately 120 keV. For power-l aw distributions of absorption depths along the lines of sight the abs orbed spectrum can indeed mimic the typical GRB spectrum. This model p redicts that (a) the spectrum should evolve monotonically from hard to soft during each energy release, which is observed in most bursts, es pecially in fast rise exponential decay bursts; (b) Fe spectral featur es near 7 keV may be present in some bursts; and (c) the ratio of burs t 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.