Vol. 549
In section 2. Astrophysical processes

Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310

Fluorescent pumping is known in many dense astrophysical sources, but the variations usually vary over many years. In this gamma-ray burst (GRB), the timescale was tens of minutes. The present study uses new analytical and numerical tools to model the time-dependent spectrum formation of a GRB afterglow induced in the circumburst medium. The photo-excitation effects of the GRB have been shown in other papers in this series by the same authors, based on spectral line variations, but this paper demonstrates - using panchromatic modeling of the broadband spectral energy distribution - that an additional mechanism is required. The authors find that the large abundance of Fe+2 requires photo-ionization of Fe+. For this purpose, the authors also present new computations of the photo-ionization cross section from the Fe II ground state to excited levels of Fe III using different atomic structure codes. The results are also relevant for the study of broad absorption line quasars and luminous blue variables, among other sources. The authors also conclude that correct modeling of the energy distribution and the time dependence of the lines requires an iron abundance above solar and a low neutral hydrogen column density. The combination of line- and continuum time-dependent modeling is a major step in the analysis of sightlines toward GRBs.