2010A&A...523A..36D

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. We analyzed medium-resolution spectroscopic observations (R=10000, corresponding to 30km/s, S/N=15-30 and wavelength range 3000-25000) of the optical afterglow of GRB090926A, taken with X-shooter at the VLT∼22 h after the GRB trigger. 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 CII, OI, SiII, FeII, and NiII-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±0.15kpc, while component II is located far away from the GRB, possibly at ∼5kpc. These values are compatible with those found for other GRBs. The hydrogen column density associated to GRB090926A is logN_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 Hα flux in emission of 9x10^–18erg/s/cm2 (i.e., a star-formation rate of 2M_☉/yr), 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 Hmolecular 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 β=0.89^+0.02_–0.02. The best fit does essentially not require any intrinsic extinction because E_B–V<0.01mag 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.