Abstract
We model the time-variable absorption of Feii, Feiii, Siii, Cii and Crii detected in Ultraviolet and Visual Echelle Spectrograph (UVES) spectra of gamma-ray burst (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 is presented in this paper, and Swift's X-Ray Telescope (XRT) 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 GRB080310. 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, as a large fraction of the ionized Feii ions (we calculate 31% using the Flexible Atomic 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 broad absorption line (BAL) quasars and η 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 GRB080310 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) in the host of GRB080310. Finally, the modelling provides indications for the presence of an additional cloud at 10 50 pc from the GRB with log N(Hi) ∼ 19 20 before the burst, which became fully ionized by the radiation released during the first few tens of minutes after the GRB.
| Original language | English |
|---|---|
| Article number | A22 |
| Journal | Astronomy and Astrophysics |
| Volume | 549 |
| DOIs | |
| Publication status | Published - 2013 |
Bibliographical note
Funding Information: P.M.V. is grateful for the support from the ESO Scientific Visitor programme in Santiago, Chile. P.R.W. and W.T.V. acknowledge support for the RAPTOR and Thinking Telescopes projects from the Laboratory Directed Research and Development programme at LANL. A.D.C. acknowledges support from the ESO DGDF 2009, 2010 and the University of Iceland Research Fund. P.J. acknowledges support by a Project Grant from the Icelandic Research Fund. The Dark Cosmology Centre is funded by the Danish National Research Foundation. The modelling performed in this paper was mostly performed on the excellent computing facilities provided by the Danish Centre for Scientific Computing (DCSC). We kindly thank Peter Laursen for the insightful discussions on Lyα scattering and Gudlaugur Johannesson for use of his 24-core work station when the DCSC servers were down. Last but not least, we are grateful for the professional assistance of the VLT staff astronomers, in particular Claudio Melo and Dominique Naef, who secured the UVES observations on which this paper is based.Other keywords
- Atomic processes
- Galaxies: ISM
- Gamma-ray burst: individual: GRB 080310
- Quasars: absorption lines
- Radiation mechanisms: thermal
- Radiative transfer