GRB 090417B and its Host Galaxy: A Step Towards an Understanding of Optically-Dark Gamma-Ray Bursts

TL;DR

This study analyzes the optically dark gamma-ray burst GRB 090417B, showing that its darkness is due to localized dust extinction in its host galaxy, providing insights into the nature of dark bursts.

Contribution

First clear identification of a host galaxy for a dark gamma-ray burst, demonstrating dust extinction as the cause of optical darkness.

Findings

Optical flux is at least 2.5 mag fainter than predicted by X-ray data.

A dense dust sheet 30-80 pc from the burst causes significant optical extinction.

The dust extinction explains the dark nature of GRB 090417B.

Abstract

GRB 090417B was an unusually long burst with a T_90 duration of at least 2130 s and a multi-peaked light curve at energies of 15-150 keV. It was optically dark and has been associated with a bright star-forming galaxy at a redshift of 0.345 that is broadly similar to the Milky Way. This is one of the few cases where a host galaxy has been clearly identified for a dark gamma-ray burst and thus an ideal candidate for studying the origin of dark bursts. We find that the dark nature of GRB 090417B cannot be explained by high redshift, incomplete observations, or unusual physics in the production of the afterglow. Assuming the standard relativistic fireball model for the afterglow we find that the optical flux is at least 2.5 mag fainter than predicted by the X-ray flux. The Swift/XRT X -ray data are consistent with the afterglow being obscured by a dense, localized sheet of dust approximately 30-80 pc from the burst along the line of sight. Our results suggest that this dust sheet imparts an extinction of A_V >~ 12 mag, which is sufficient to explain the missing optical flux. GRB 090417B is an example of a gamma-ray burst that is dark due to the localized dust structure in its host galaxy.