Line Emission from Gamma‐Ray Burst Environments

The time and angle dependent line and continuum emission from a dense torusaround a cosmological gamma-ray burst source is simulated, taking into accountphotoionization, collisional ionization, recombination, and electron heatingand cooling due to various processes. The importance of the hydrodynamicalinteraction between the torus and the expanding blast wave is stressed. Due tothe rapid deceleration of the blast wave as it interacts with the dense torus,the material in the torus will be illuminated by a drastically different photonspectrum than observable through a low-column-density line of sight, and willbe heated by the hydrodynamical interaction between the blast wave and thetorus. A model calculation to reproduce the Fe K-alpha line emission observedin the X-ray afterglow of GRB 970508 is presented. The results indicate that ~10^{-4} solar masses of iron must be concentrated in a region of less than10^{-3} pc. The illumination of the torus material due to the hydrodynamicinteraction of the blast wave with the torus is the dominant heating andionization mechanism leading to the formation of the iron line. These resultssuggest that misaligned GRBs may be detectable as X-ray flashes with pronouncediron emission line features.Comment: Accepted for publication in ApJ. Updated recombination rate data; discussion on element abundances added; references update