The host of GRB 060206: kinematics of a distant galaxy

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: cthoene@dark-cosmology.dk
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago 19, Chile
⁴ Dept. of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁵ Instituto de Astrofísica de Andalucía, (IAA-CSIC), Apdo. 3004, 18080 Granada, Spain
⁶ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁷ Herschel Science Operations Center, INSA, ESAC, European Space Agency, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
⁸ University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

Received: January 1900
Accepted: 28 May 2008

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/N to allow for these kinds of studies.

Aims. We investigate the state and properties of the interstellar medium in the host of GRB 060206 at 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 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 was detected as well. The host is detected in deep HST imaging with  ± 0.19 mag and a 3σ upper limit of 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.