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]
