Non-variability of intervening absorbers observed in the UVES spectra of the ‘naked-eye’ GRB080319

The aim of this paper is to investigate the properties of the intervening absorbers lying along the line of sight of GRB080319B through the analysis of its optical absorption features. For this purpose, we analyse Ultraviolet and Visual Echelle Spectrograph (UVES)/Very Large Telescope multi-epoch, high-resolution spectroscopic observations (R= 40 000, corresponding to 7.5 km s−1) of the optical afterglow of GRB080319B (z= 0.937), detected by Swift. Thanks to the rapid response mode (RRM), we observed the afterglow just 8 min 30 s after the gamma-ray burst (GRB) onset when the magnitude was R∼ 12. This allowed us to obtain the best signal-to-noise ratio (S/N), high-resolution spectrum of a GRB afterglow ever (S/N per resolution element of ∼50). Two further RRM and target of opportunity observations were obtained starting 1.0 and 2.4 h after the event, respectively. Four Mg ii absorption systems lying along the line of sight to the afterglow have been detected in the redshift range 0.5 < z < 0.8, most of them showing a complex structure featuring several components. Absorptions due to Fe ii, Mg i and Mn ii are also present; they appear in four, two and one intervening absorbers, respectively. One out of four systems show an Mg iiλ2796 rest-frame-equivalent width larger than 1 Å. This is in agreement with the excess of strong Mg ii absorbers compared to quasars, with dn/dz= 0.9, approximately four times larger than the value observed along quasar lines of sight. In addition, the analysis of multi-epoch, high-resolution spectra allowed us to exclude a significant variability in the column density of the single components of each absorber. In particular, 17 out of 21 Mg ii components belonging to the four absorbers do not vary at the 3σ confidence level, and the column densities of the remaining four are saturated and cannot be reliably measured. Combining this result with estimates of the size of the emitting region, we can reject the hypothesis that the difference between GRB and quasi-stellar object Mg ii absorbers is due to a different size of the emitting regions.