Issue |
A&A
Volume 545, September 2012
|
|
---|---|---|
Article Number | A64 | |
Number of page(s) | 18 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201218884 | |
Published online | 10 September 2012 |
Rapid-response mode VLT/UVES spectroscopy of super iron-rich gas exposed to GRB 080310
Evidence of ionization in action and episodic star formation in the host⋆,⋆⋆
1
Centre for Astrophysics and Cosmology, Science Institute, University of
Iceland,
Dunhagi 5,
107
Reykjavik,
Iceland
e-mail: annalisa.decia@gmail.com
2
European Southern Observatory, Alonso de Córdova 3107, Casilla 19001,
Santiago 19,
Chile
3
STScI, 3700 San Martin Drive, Baltimore, MD
21218,
USA
4
Institut d’Astrophysique de Paris, Université Paris
6, 98bis Boulevard
Arago, 75014
Paris,
France
5
Dark Cosmology Centre, Niels Bohr Institute, University of
Copenhagen, 2100
Copenhagen,
Denmark
Received: 25 January 2012
Accepted: 3 July 2012
We analyse high-resolution near-UV and optical spectra of the afterglow of GRB 080310, obtained with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT/UVES), to investigate the circumburst environment and the interstellar medium of the gamma-ray burst (GRB) host galaxy. The VLT rapid-response mode (RRM) enabled the observations to start only 13 min after the Swift trigger and a series of four exposures to be collected before dawn. A low neutral-hydrogen column-density (log N(H i) = 18.7) is measured at the host-galaxy redshift of z = 2.42743. At this redshift, we also detect a large number of resonance ground-state absorption lines (e.g., C ii, Mg ii, Al ii, Si ii, Cr ii, C iv, Si iv), as well as time-varying absorption from the fine-structure levels of Fe ii. Time-varying absorption from a highly excited Fe iii energy level (7S3), giving rise to the so-called UV 34 line triplet, is also detected, for the first time in a GRB afterglow. The Cr ii ground-state and all observed Fe ii energy levels are found to depopulate with time, whilst the Fe iii7S3 level is increasingly populated. This absorption-line variability is clear evidence of ionization by the GRB, which is for the first time conclusively observed in a GRB afterglow spectrum. We derive ionic column densities at each epoch of observations by fitting absorption lines with a four-component Voigt-profile model. We perform CLOUDY photo-ionization modelling of the expected pre-burst ionic column densities, to estimate that, before the onset of the burst, [C/H] = −1.3 ± 0.2, [O/H] < −0.8, [Si/H] = −1.2 ± 0.2, [Cr/H] = +0.7 ± 0.2, and [Fe/H] = +0.2 ± 0.2 for the integrated line profile, indicating strong overabundances of iron and chromium. For one of the components, we observe even more extreme ratios of [Si/Fe] ≤ −1.47 and [C/Fe] ≤ −1.74. These peculiar chemical abundances cannot easily be explained by current models of supernova yields. They are indicative of a low dust content, whilst dust destruction may also contribute to the marked Fe and Cr overabundances. The overall high iron enhancement along the line-of-sight suggests that there has been negligible recent star formation in the host galaxy. Thus, the occurrence of a GRB indicates that there has been episodes of massive star formation in the GRB region.
Key words: gamma-ray burst: individual: GRB 080310 / quasars: absorption lines / galaxies: ISM / galaxies: abundances
Based on Target-Of-Opportunity observations carried out in service mode using the Very Large Telescope Rapid-Response Mode under programme ID 080.D-0526, P.I. Vreeswijk, with the Ultraviolet and Visual Echelle Spectrograph (UVES) installed at the Nasmyth-B focus of the VLT, Unit 2, Kueyen, operated by the European Southern Observatory (ESO) on Cerro Paranal in Chile.
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2012
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.