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

Abstract

We study the environment and intervening absorbers of the gamma‐ray burst GRB 090926A through analysis of X‐shooter optical spectra of its afterglow taken ∼22 hr after the 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 several excited absorption features. No host galaxy emission lines, molecular absorption features nor diffuse interstellar bands are detected in the spectrum. The line of sight of GRB 090926A presents four weak intervening absorption systems in the redshift range 1.24<z<1.95. The Hydrogen column density associated to GRB 090926A is logNH/cm−2 = 21.60±0.07, and the metallicity of the host galaxy is in the range [X/H] = 4.2×10−3−1.4×10−2 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. The continuum has been fitted assuming a power‐law spectrum, with a spectral index of β = 0.890.02+0.02. The best fit does not essentially require any intrinsic extinction since EB−V<0.01 mag adopting a SMC extinction curve. We derive information on the distance between the host absorbing gas and the site of the GRB explosion. The distance of the redmost component is found to be 2.40±0.15 kpc, while the bluemost one is located far away from the GRB, possibly at ∼5 kpc.