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arXiv · 1207.6102

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

A. De Cia·C. Ledoux·A. J. Fox·P. M. Vreeswijk·A. Smette·P. Petitjean·G. Bjornsson·J. P. U. Fynbo·J. Hjorth·P. Jakobsson
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Abstract

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 minutes after the Swift trigger and a series of four exposures to be collected before dawn. A low neutral-hydrogen column-density (log N (HI) = 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., CII, MgII, AlII, SiII, CrII, CIV, SiIV), as well as time-varying absorption from the fine-structure levels of FeII. Time-varying absorption from a highly excited FeIII energy level (7S3), giving rise to the so-called UV34 line triplet, is also detected, for the first time in a GRB afterglow. The CrII ground-state and all observed FeII energy levels are found to depopulate with time, whilst the FeIII 7S3 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 \pm 0.2, [O/H] < -0.8, [Si/H] = -1.2 \pm 0.2, [Cr/H] = +0.7 \pm 0.2, and [Fe/H] = +0.2 \pm 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] \leq -1.47 and [C/Fe] \leq -1.74. [abridged]

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A. De Cia, C. Ledoux, A. J. Fox, P. M. Vreeswijk, A. Smette, P. Petitjean, G. Bjornsson, J. P. U. Fynbo, J. Hjorth, P. Jakobsson. 2012-07-25. 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. https://doi.org/10.1051/0004-6361%2F201218884

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