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 GRB 090926A through analyzing optical spectra of its afterglow. Methods. We analyzed medium-resolution spectroscopic observations (R = 10000, corresponding to 30 km s-1, S/N = 15−30 and wavelength range 3000−25000) of the optical afterglow of GRB 090926A, taken with X-shooter at the VLT ~ 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, O i, Si ii, Fe ii, 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 ~5 kpc. These values are compatible with those found for other GRBs. The hydrogen column density associated to GRB 090926A is log NH/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 Hα flux in emission of 9 × 10-18 erg s-1 cm-2 (i.e., a star-formation rate of 2 M⊙ 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 < 7 × 10-7. The continuum has been fitted assuming a power-law spectrum, with a spectral index of . 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 GRB 090926A presents four weak intervening absorption systems in the redshift range 1.24 < z < 1.95.