Probing the complex environments of GRB host galaxies and intervening systems: high resolution spectroscopy of GRB050922C

Abstract

Aims. The aim of this paper is to investigate the environment of gamma ray bursts (GRBs) and the interstellar matter of their host galaxies. Methods. We use to this purpose high resolution spectroscopic observations of the afterglow of GRB050922C, obtained with UVES/VLT∼ 3.5 hours after the GRB event. Results. We found that, as for most high resolution spectra of GRBs, the spectrum of the afterglow of GRB050922C is complex. At least seven components contribute to the main absorption system at z = 2.1992. The detection of lines of neutral elements like MgI and the detection of fine-structure levels of the ions FeII, S iII and CII allows us to separate components in the GRB ISM along the line of sight. Moreover, in addition to the main system, we have analyzed the five intervening systems between z = 2.077 and z = 1.5664 identified along the GRB line of sight. Conclusions. GRB afterglow spectra are very complex, but full of information. This can be used to disentangle the contribution of the different parts of the GRB host galaxy and to study their properties. Our metallicity estimates agree with the scenario of GRBs exploding in low metallicity galaxies