Abstract
Context. GRB afterglow spectra are sensitive probes of interstellar matter along the line-of-sight in their host galaxies, as well as in intervening galaxies. The rapid fading of GRBs makes it very difficult to obtain spectra of sufficient resolution and S/ Nt o allow for these kinds of studies. Aims. We investigate the state and properties of the interstellar medium in the host of GRB 060206 at z = 4.048 with a detailed study of groundstate and finestructure absorption lines in an early afterglow spectrum. This allows us to derive conclusions on the nature and origin of the absorbing structures and their connection to the host galaxy and/or the GRB. Methods. We used early (starting 1.6 h after the burst) WHT/ISIS optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 detecting a range of metal absorption lines and their finestructure transitions. Additional information is provided by the afterglow lightcurve. The resolution and wavelength range of the spectra and the bright afterglow have facilitated a detailed study and fitting of the absorption line systems in order to derive column densities. We also used deep imaging to detect the host galaxy and probe the nature of an intervening system at z = 1.48 seen in absorption in the afterglow spectra. Results. We detect four discrete velocity systems in the resonant metal absorption lines, best explained by shells within and/or around the host created by starburst winds. The finestructure lines have no less than three components with strengths decreasing from the redmost components. We therefore suggest that the finestructure lines are best explained as being produced by UV pumping from which follows that the redmost component is the one closest to the burst where N v was detected as well. The host is detected in deep HST imaging with F814WAB = 27.48 ± 0.19 mag and a 3σ upper limit of H = 20.6 mag (Vega) is achieved. A candidate counterpart for the intervening absorption system is detected as well, which is quite exceptional for an absorber in the sightline towards a GRB afterglow. The intervening system shows no temporal evolution as claimed by Hao et al. (2007, ApJ, 659, 99), which we prove from our WHT spectra taken before and Subaru spectra taken during those observations.
Highlights
In recent years, gamma-ray bursts (GRBs) have been detected out to very high redshifts with four at z > 5 currently (Jakobsson et al 2006a; Cenko et al 2006; Ruiz-Velasco et al 2007; Haislip et al 2006; Kawai et al 2006)
The intervening system shows no temporal evolution as claimed by Hao et al (2007, ApJ, 659, 99), which we prove from our William Herschel Telescope (WHT) spectra taken before and Subaru spectra taken during those observations
In our first paper on the dataset presented here (Fynbo et al 2006, hereafter Paper I), we showed initial results of the WHT and NOT spectra of the afterglow of GRB 060206, focusing on the metallicity that we compared to a large sample of QSO absorber metallicities and the sample of known GRB afterglow metallicities
Summary
GRBs have been detected out to very high redshifts with four at z > 5 currently (Jakobsson et al 2006a; Cenko et al 2006; Ruiz-Velasco et al 2007; Haislip et al 2006; Kawai et al 2006). The line-of-sight metallicities derived from afterglow spectroscopy are higher than the mean QSO absorber metallicity as a function of redshift (Fynbo et al 2006; Prochaska et al 2007). In our first paper on the dataset presented here (Fynbo et al 2006, hereafter Paper I), we showed initial results of the WHT and NOT spectra of the afterglow of GRB 060206, focusing on the metallicity that we compared to a large sample of QSO absorber metallicities and the sample of known GRB afterglow metallicities In this second paper we analyze the full set of absorption lines and add a more detailed analysis of the properties of the host from the photometric detection of a possible host galaxy and broadband afterglow properties.
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