VLT/X-shooter spectroscopy of the GRB 090926A afterglow

Aims. The aim of this paper is to study the environment and intervening absorbers of the gamma-ray burst GRB090926A through analyzing optical spectra of its afterglow. Methods. We analyzed medium-resolution spectroscopic observations (R = 10 000, corresponding to 30 km s(-1), S/N = 15-30 and wavelength range 3000-25 000) of the optical afterglow of GRB090926A, taken with X-shooter at the VLT similar to 22 h after the GRB trigger. Results. The spectrum shows that the ISM in the GRB host galaxy at z = 2.1071 is rich in absorption features, with two components contributing to the line profiles. In addition to the ground state lines, we detect C II, OI, Si II, FeII, and Ni II-excited absorption features, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The distance of component I is found to be 2.40 +/- 0.15 kpc, while component II is located far away from the GRB, possibly at similar to 5 kpc. These values are compatible with those found for other GRBs. The hydrogen column density associated to GRB 090926A is log N-H/cm(-2) = 21.60 +/- 0.07, and the metallicity of the host galaxy is in the range [X/H] = -2.5 to -1.9 with respect to the solar values, i.e., among the lowest values ever observed for a GRB host galaxy. A comparison with galactic chemical evolution models has suggested that the host of GRB090926A is likely to be a dwarf-irregular galaxy. No emission lines were detected, but a Ha flux in emission of 9 x 10(-18) erg s(-1) cm(-2) (i.e., a star-formation rate of 2 M-circle dot yr(-1)), which is typical of many GRB hosts, would have been detected in our spectra, and thus emission lines are well within the reach of X-shooter. We put an upper limit to the H molecular fraction of the host galaxy ISM, which is f < 7x10(-7). The continuum has been fitted assuming a power-law spectrum, with a spectral index of beta = 0.89(-0.02)(+0.02) . The best fit does essentially not require any intrinsic extinction because EB-V < 0.01 mag adopting a SMC extinction curve. Finally, the line of sight of GRB090926A presents four weak intervening absorption systems in the redshift range 1.24 < z < 1.95.