Variable Ly alpha sheds light on the environment surrounding GRB 090426

TL;DR

This study uses variable Ly alpha absorption to investigate the environment around GRB 090426, revealing highly ionized nearby material and suggesting the burst occurred in an isolated star-forming region near interacting galaxies.

Contribution

First observation of Ly alpha absorption variability around a GRB, providing insights into the local ionized environment within 80 pc of the burst.

Findings

Detected almost fully ionized medium with low neutral hydrogen column density.

Observed Ly alpha absorption line variations for the first time in a GRB.

Probed material from the burst's vicinity at approximately 80 parsecs.

Abstract

Long duration gamma-ray bursts are commonly associated with the deaths of massive stars. Spectroscopic studies using the afterglow as a light source provide a unique opportunity to unveil the medium surrounding it, probing the densest region of their galaxies. This material is usually in a low ionisation state and at large distances from the burst site, hence representing the normal interstellar medium in the galaxy. Here we present the case of GRB 090426 at z=2.609, whose optical spectrum indicates an almost fully ionised medium together with a low column density of neutral hydrogen. For the first time, we also observe variations in the Ly alpha absorption line. Photoionisation modeling shows that we are probing material from the vicinity of the burst (~80 pc). The host galaxy is a complex of two luminous interacting galaxies, which might suggest that this burst could have occurred in an isolated star-forming region outside its host galaxy created in the interaction of the two galaxies.