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

We model the time-variable absorption of FeII, FeIII, SiII, CII and CrII detected in UVES spectra of GRB 080310, with the afterglow radiation exciting and ionizing the interstellar medium in the host galaxy at a redshift of z=2.42743. To estimate the rest-frame afterglow brightness as a function of time, we use a combination of the optical VRI photometry obtained by the RAPTOR-T telescope array – which are presented in this paper – and Swift’s X-Ray Telescope observations. Excitation alone, which has been successfully applied for a handful of other GRBs, fails to describe the observed column-density evolution in the case of GRB 080310. Inclusion of ionization is required to explain the column-density decrease of all observed FeII levels (including the ground state 6D9/2) and increase of the FeIII 7S3 level. The large population of ions in this latter level (up to 10% of all FeIII) can only be explained through ionization of FeII, whereby a large fraction of the ionized FeII ions – we calculate 31% using the Flexible Atomic (FAC) and Cowan codes – initially populate the 7S3 level of FeIII rather than the ground state. This channel for producing a significant FeIII 7S3 level population may be relevant for other objects in which absorption lines from this level – the UV34 triplet – are observed, such as BAL quasars and Eta Carinae. This provides conclusive evidence for time-variable ionization in the circumburst medium, which to date has not been convincingly detected. However, the best-fit distance of the neutral absorbing cloud to the GRB is 200–400 pc, i.e. similar to GRB-absorber distance estimates for GRBs without any evidence for ionization. We find that the presence of time-varying ionization in GRB 080310 is likely due to a combination of the super-solar iron abundance ([Fe/H]=+0.2) and the low HI column density (log N(HI)=18.7). [abridged]

💡 Research Summary

This paper presents a comprehensive time‑dependent model of the absorption‑line variability observed in the afterglow spectra of GRB 080310. Using high‑resolution UVES data, the authors track the column densities of Fe II, Fe III, Si II, C II, and Cr II from a few minutes up to several hours after the burst. To feed the model they construct the rest‑frame afterglow luminosity as a function of time by combining optical VRI photometry obtained with the RAPTOR‑T telescope array (presented here for the first time) and Swift X‑Ray Telescope observations.

Traditional afterglow‑excitation models, which consider only UV pumping and collisional excitation, have successfully explained line‑variability in a handful of previous GRBs. In the case of GRB 080310, however, excitation alone cannot reproduce the simultaneous decrease of all Fe II levels—including the ground state 6D₉/₂—and the increase of the Fe III 7S₃ level (the UV34 triplet). The authors therefore extend the model to include photo‑ionization of the surrounding interstellar medium.

A key finding is that a substantial fraction of Fe II atoms that are ionized by the afterglow photons do not immediately cascade to the Fe III ground state. Using two independent atomic‑structure codes, the Flexible Atomic Code (FAC) and the Cowan code, they calculate that about 31 % of the ionized Fe II ions populate the Fe III 7S₃ level directly. This channel naturally explains the observed high population of the 7S₃ level—up to 10 % of all Fe III ions—without invoking exotic excitation mechanisms.

The best‑fit model places the neutral absorbing cloud at a distance of 200–400 pc from the GRB, a range comparable to distances inferred for GRBs that show no ionization signatures. The authors argue that the detection of time‑varying ionization in GRB 080310 is facilitated by two unusual environmental properties: a super‑solar iron abundance (