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

TL;DR

This study uses VLT/X-shooter spectroscopy to analyze the environment and intervening absorbers of GRB 090926A, revealing a low-metallicity host galaxy with detailed absorption features and estimating the distance of absorbing gas from the burst.

Contribution

First detailed spectroscopic analysis of GRB 090926A's environment, providing insights into its host galaxy's properties and the location of absorbing gas relative to the burst.

Findings

Host galaxy has very low metallicity ([X/H] = 3.2x10^{-3} to 1.2x10^{-2})

Hydrogen column density is log N_H = 21.60 +/- 0.07

Absorbing gas components are located at 2.4 and ~5 kpc from the GRB

Abstract

The aim of this paper is to study the environment and intervening absorbers of the gamma-ray burst GRB 090926A through analysis of optical spectra of its afterglow. We analyze medium resolution spectroscopic observations (R=10 000, corresponding to 30 km/s, S/N=15 - 30 and wavelength range 3000-25000) of the optical afterglow of GRB 090926A, taken with X-shooter at the VLT ~ 22 hr 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 C II, O I, Si II, Fe II and Ni II excited absorption features. No host galaxy emission lines, molecular absorption features nor diffuse interstellar bands are detected in the spectrum. 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] =3.2X10^{-3}-1.2X10^{-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. We put an upper limit to the Hydrogen molecular fraction of the host galaxy ISM, which is f < 7X10^{-7}. We 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 that found for other GRBs.